Bài toán tìm tập hợp điểm và cực trị của số phức – Diệp Tuân Toán 12

Bài toán tìm tập hợp điểm và cực trị của số phức – Diệp Tuân Toán 12 được sưu tầm và soạn thảo dưới dạng file PDF để gửi tới các bạn học sinh cùng tham khảo, ôn tập đầy đủ kiến thức, chuẩn bị cho các buổi học thật tốt. Mời bạn đọc đón xem!

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A-L THUY󰈸T
I. ĐIỂM BIỄU DIỄN CỦA SỐ PHỨC.
1. Định nghĩa: Đim
( ; )M a b
trong h trc tọa độ
vuông góc ca mt phẳng được gọi là điểm biu
din ca s phc
.z a bi
2. Tính cht.
Các điểm
( ; ), ( ; )M a b M a b
biu din
z
đối xng vi nhau qua trc hoành
.Ox
Ví d 1. Quan sát hình v bên cnh, ta có:
Đim
(2;1)A
biu din cho s phc
1
2.zi
Đim
(....;....)B
biu din cho s phc
2
...........z
Đim
(....;....)C
biu din cho s phc
3
...........z
Đim
(....;....)D
biu din cho s phc
4
...........z
Đim
(....;....)E
biu din cho s phc
5
...........z
Đim
(....;....)F
biu din cho s phc
6
...........z
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Ví d 2. Gi
,,A B C
lần lượt là điểm biu din ca các s phc
1
3 2 ,zi
2
23zi
,
3
54zi
.
1). Chng minh
,,A B C
là ba đỉnh của tam giác. Tính chu vi tam giác đó.
2). Gi
D
là điểm biu din ca s phc
. Tìm
z
để
ABCD
là hình bình hành.
3). Gi
E
là điểm biu din ca s phc
'z
. Tìm
'z
sao cho tam giác
AEB
vuông cân ti
E
.
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BI 2 . TẬP HỢP ĐIỂM – CỰC TRỊ CỦA SỐ PHỨC
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Ví d 3. Gi
1 2 3 4
, , ,A A A A
lần lượt là biu din hnh hc ca các s phc
12
1 3 , 3 2 ,z i z i
34
5 , 4 5z i z i
.
1). Tính đ dài các đoạn
1 2 1 3 1 4
, , A A A A A A
.
2). Tm s phc có biu din là đim
M
sao cho
1 2 4
A A A M
là hnh bnh hành.
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d 4. Gi các đim
,,A B C
trong mt phng phc lần lượt theo th t biu din các s
4
1
i
i
,
1 1 2 ,ii
26
3
i
i
.
1). Chng minh
ABC
là tam giác vuông cân
2). Tìm s phc biu din bởi điểm
D
sao cho
ABCD
là hình vuông.
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3. Mt s bài toán tìm tp hp điểm phương pháp.
Bài toán 1. Tập hợp một đường mt đường thẳng
0Ax By C
1. Nhận dạng trắc nghiệm.
Khi gp gi thiết s phc có dng
z a bi z c di
1.
Ta nghĩ ngay tập hợp biễu diễn của
số phức
điểm
;M x y
nằm trên đường thẳng
0Ax By C
(
đường trung trực của đoạn
AB
với
, , ,A a b B c d
Đặt biệt: Khi biến đổi điều kiện của giả thiết về:
0x
là trục tung.
0y
trục hoành.
2. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
.,z x y i x y
c 2. Biến đổi điều kin
1
để tìm mi liên ca
x
y
giống như các dng trên.
c 3. Kết lun.
3. Bài tập minh họa.
Bài tp 1.Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
thỏa mãn điều
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kin:
2z i z
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Bài tp 2.Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
thỏa mãn điều
kin:
2
z
là s o.
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Bài tp 3.Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
thỏa mãn điều
kin:
23z i z i
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Bài tp 4. Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
thỏa mãn điều
kin:
2
2
zz
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Bài tp 5. Tm tp hp nh󰊀ng điểm
M
biu din s phc
tha:
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1).
43zi
là s thc. 2).
32z i z i
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Bài tp 6. Tìm tp hợp điểm
M
biu din s phc
tha
23
3
zi
zi


là một số thực dương.
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Bài tp 7. Cho số phức
thỏa mãn
2 2 3 2 1 2 .z i i z
Tập hợp các điểm
M
biểu diễn số
phức
trong mặt phẳng tọa độ
Oxy
là đường thẳng có phương trnh nào sau đây ?
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4. Câu hỏi trắc nghiệm.
Mc độ. Nhn biết
u 1.(THPT Lý Thường Kit 2019) Cho hai s phc
1
23zi
,
2
1zi
. Điểm biu din s phc
12
2zz
trên mt phng tọa độ là.
A.
0; 5
. B.
4; 1
. C.
0; 1
. D.
5;0
.
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u 2.(Triu Thái Vĩnh Phúc Ln 3) Tìm tọa độ đim
M
trong mt phng
Oxy
điểm biu din
s phc
34zi
.
A.
3; 4 .M
B.
3;4 .M
C.
3;4 .M
D.
3; 4 .M 
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u 3.ng Thành Nam) Trong hình v bên, điểm
P
biu din
s phc
1
z
, điểm
Q
biu din s phc
2
z
. Mệnh đề nào dưới đây
đúng?
A.
12
zz
. B.
12
5zz
. C.
12
5zz
. D.
12
zz
.
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u 4.(S GD & ĐT Nam Định 2019) Trong mt phng tọa độ
Oxy
, điểm biu din s phc
45zi
có tọa độ
A.
4;5
. B.
4; 5
. C.
4; 5
. D.
5; 4
.
Li gii
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u 5.(THPT Thanh Chương 2019) Gi
M
,
N
lần lượt điểm biu din hình hc các s phc
2zi
45wi
. Tọa độ trung điểm
I
của đoạn thng
MN
A.
2;3I
. B.
4;6I
. C.
3;2I
. D.
6 ;4I
.
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u 6.(THPT Nguyn Tt Thành 2019) Đim
M
trong hình v
bên điểm biu din s phc
z
. Tìm phn thc phn o ca
s phc
z
.
A. Phn thc là
1
và phn o là
2i
.
B. Phn thc là
2
và phn o là
1
.
C. Phn thc là
2
và phn o là
i
.
D. Phn thc là
1
và phn o là
2
.
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u 7.(THPT KonTum 2019) Cho s phc
z
có biu din hình hc trong mt phng tọa độ
Oxy
đim
3; 4M
. Môđun của
z
bng
A.
25
. B.
5
. C.
1
. D.
5
.
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u 8.(THPT Gia Lc 2019) Cho số phức
z
có số phức liên hợp
z
. Gọi
M
M
tương ứng là
điểm biểu diễn hnh học của
z
z
. Hãy chọn mệnh đề đúng.
A.
M
M
đối xng nhau qua trc o. B.
M
M
trùng nhau.
C.
M
M
đối xng nhau qua trc thc. D.
M
M
đối xng nhau qua gc tọa độ.
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u 9.(THPT Hùng Vương 2019) Trong hnh vẽ bên, điểm
M
biểu
diễn số phức
z
. Số phức
z
là:
A.
12i
. B.
2 i
. C.
12i
. D.
2 i
.
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u 10. Cho s phc tha mãn Hỏi điểm biu
din ca là điểm nào trong các điểm
, , ,M N P Q
hnh dưới?
A. Đim
P
.
B. Đim
Q
.
C. Đim
M
.
D. Đim
N
.
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u 11.(Chuyên Đại Hc Vinh 2019) Cho s phc .
Trong hnh bên điểm biu din s phc
A. . B. . C. . D. .
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z
(2 ) 7 .i z i
z
2zi
z
M
Q
P
N
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u 12.(Đặng Thành Nam) Điểm nào trong hnh vẽ bên là điểm
biểu diễn số phức
2zi
?
A.
N
. B.
P
.
C.
M
. D.
Q
.
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u 13.(THPT Triu Ti 2019) Gi
, , A B C
là điểm biu din cho các s phc sau
1
1 3 ,zi
23
3 2 , 4z i z i
. Tìm kết luận đúng nhất?
A. Tam giác
ABC
cân. B. Tam giác
ABC
vuông cân.
C. Tam giác
ABC
đều. D. Tam giác
ABC
vuông.
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u 14.(THPT ISCHOOL Nha Trang) Cho s phc
25
34
z
i
. Điểm biu din hình hc s phc liên
hp ca
z
trong mt phng
Oxy
A.
3; 4M
. B.
2; 3N
. C.
3; 2P
. D.
3;4Q
.
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u 15.(Chuyên Đại Hc Vinh 2019) Cho s phc tha mãn . Điểm biu din s
phc có tọa độ
A. . B. . C. . D. .
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u 16. ng Thành Nam) Các điểm
A
B
trong hình v ln
ợt là điểm biu din ca các s phc
12
,zz
. S phc
12
zz
A.
2 i
. B.
13i
. C.
2 i
. D.
13i
.
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z
2 6 2z z i
z
2; 2
2; 2
2;2
2;2
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u 17.(Tp Chí Toán Hc 2019) Trong mt phng
Oxy
, gi
M
,
N
theo th t các điểm biu
din cho s phc
z
z
(vi
0z
). Mệnh đề o dưới đây đúng?
A.
M
N
đối xng vi nhau qua trc
Ox
.
B.
M
N
đối xng vi nhau qua trc
Oy
.
C.
M
N
đối xng với nhau qua đường phân giác ca góc phần tư thứ nht.
D.
M
N
đối xng với nhau qua đường phân giác ca góc phần tư thứ hai.
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u 18.(THPT Hai Trưng 2019) Điểm nào trong hnh vẽ dưới
đây là điểm biểu diễn số phức liên hợp của số phức
32zi
?
A.
M
. B.
N
. C.
Q
. D.
P
.
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Mc độ 2. Thông Hiu
u 19.(THPT Kim Liên 2017) Trên mặt phẳng tọa độ, các điểm
A
,
B
,
C
theo thứ tự biểu diễn
các số phức
23i
,
3 i
,
12i
. Trng tâm
G
ca tam giác
ABC
biu din s phc
z
. Tìm
z
A.
1zi
. B.
22zi
. C.
22zi
. D.
1zi
.
Li gii
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u 20.(THPT Chuyên Thái Nguyên 2019) Trong mặt phẳng
,Oxy
gọi
,,A B C
lần ợt các điểm
biểu diễn số phức
1
3,zi
2
2 2 ,zi
3
5zi
. Gọi
G
trọng tâm của tam giác
ABC
. Khi đó
điểm
G
biểu diễn số phức
A.
1zi
. B.
12zi
. C.
12zi
. D.
2zi
.
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u 21.(THPT Đô Lương 2019) Biết
4; 3M
điểm biu din s phc
z
trên mt phng phc.
Khi đó điểm nào sau đây biểu din s phc
wz
?
A.
4; 3N 
. B.
3; 4R
. C.
4; 3Q
. D.
4;3P
.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 22.(THPT chuyên Quý Đôn 2017) Số nào sau đây số đối của số phức
, biết
phần
thực dương thỏa mãn
2z
và trong mặt phẳng phức th
z
có điểm biểu diễn thuộc đường thẳng
30yx
.
A.
13i
. B.
13i
. C.
13i
. D.
13i
.
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u 23.(Cụm Trường Sóc n Linh 2019) Cho các số phức
12
,zz
điểm biểu diễn trên mặt
phẳng tọa độ lần lượt là
,MN
. Gọi
P
là trung điểm của
MN
, khi đó
P
biểu diễn số phức
A.
12
2
zz
. B.
12
2
zz
. C.
12
zz
. D.
12
zz
.
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u 24.(THPT Thanh Chương 2019) Gi
A
,
B
,
C
lần lượt điểm biu din hình hc ca các s
phc
1
12zi
,
2
1zi
3
34zi
. Điểm
G
trng tâm
ABC
là điểm biu din ca s phc
nào sau đây?
A.
1zi
B.
33zi
. C.
12zi
. D.
1zi
.
Li gii
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u 25.(THPT Chuyên Khiết 2017) Gọi
A
,
B
,
C
lần lýợt các ðiểm biểu diễn của số phức
1
13zi
,
2
32zi
,
3
4zi
trong hệ tọa ðộ
Oxy
. Hãy chọn kết lun ðúng nhất.
A. Tam giác
ABC
vuông cân. B. Tam giác
ABC
đều.
C. Tam giác
ABC
vuông. D. Tam giác
ABC
cân.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 26.(THPT KonTum 2019) Cho các s phc
1
32zi
,
2
14zi
3
1zi
biu din
hình hc trong mt phng tọa độ
Oxy
lần lượt các điểm
A
,
B
,
C
. Din tích tam giác
ABC
bng:
A.
2 17
. B. 12. C.
4 13
. D. 9.
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u 27.(S GD & ĐT Cần Thơ 2019) Trong mt phng
Oxy
, gi
M
điểm biu din ca s phc
34zi
M
là điểm biu din ca s phc
1
2
i
zz
. Din tích ca tam giác
OMM
bng
A.
25
4
. B.
25
2
. C.
15
4
. D.
15
2
.
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u 28.(THPT chuyên Lam n 2017) Cho các điểm
, ,CAB
nằm trong mặt phẳng phức lần lượt
biểu diễn các số phức
13i
,
22i
,
17i
. Gọi
D
điểm sao cho tứ giác
ABCD
hình bình
hành. Điểm
D
biểu diễn số phức nào trong các số phức sau đây?
A.
28zi
. B.
46zi
. C.
46zi
. D.
28zi
.
Li gii
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u 29.(THPT Kinh n 2019) Biết rằng ba điểm
A
,
B
,
C
lần lượt các điểm biu din hình
hc ca s phc
1
12zi
,
2
3zi
;
3
22 zi
. Tìm tọa độ đỉnh th của hình bình hành
ABCD
.
A.
6; 5D
. B.
6; 3D
. C.
4; 3D
. D.
4; 5D
.
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Li gii
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u 30.(Đề tham khảo 2017) Trong mặt phẳng tọa độ, điểm
M
điểm biểu diễn củasố phức
(như hnh vẽ bên). Điểm o
trong hnh vẽ là điểm biểu diễn của số phức
2z
?
A. Điểm
N
B. Điểm
Q
C. Điểm
E
D. Điểm
P
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u 31.(THPT Ngô Quyn Hi Phòng 2019) Trong mt phng
phc, cho s phc
z
điểm biu din
M
. Biết rng s phc
1
w
z
đưc biu din bi mt trong bốn điểm
, , ,N P Q R
như
hình v bên. Hỏi điểm biu din ca
w
là điểm nào?
A.
N
. B.
Q
. C.
P
. D.
R
.
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u 32.(THPT Chuyên Nguyn Quang Diêu 2019) Gi
M
điểm biu din s phc
z
,
N
điểm
biu din s phc
w
trong mt phng tọa độ. Biết
N
điểm đối xng vi
M
qua trc
Oy
(
M
,
N
không thuc các trc tọa độ). Mệnh đề nào sau đây đúng?
A.
wz
. B.
wz
. C.
wz
. D.
wz
.
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x
y
R
Q
P
N
M
O
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u 33.(Chuyên Đại Học Vinh 2017) Cho số phức
điểm biểu
diễn
M
. Biết rằng số phức
1
w
z
được biểu diễn bởi một
trong bốn điểm
P
,
Q
,
R
,
S
nhnh vẽ bên. Hỏi điểm biểu
diễn của
w
là điểm nào?
A.
S
. B.
P
. C.
Q
. D.
R
.
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u 34.(THPT Chuyên Đại Học Vinh) Cho số phức
thỏa mãn
2
2
z
điểm
A
trong hnh vẽ bên điểm biểu diễn của
.
Biết rằng trong hnh vẽ bên, điểm biểu diễn của số phức
1
w
iz
một trong bốn điểm
, , ,M N P Q
. Khi đó điểm biểu diễn của số
phức
w
là.
A. điểm
Q
. B. điểm
P
. C. điểm
M
. D. điểm
N
.
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Mc độ 3. Vn dng
u 35.(THPT chuyên Nguyễn Trãi) Cho số phức
thỏa mãn
2
1 z
là số thực. Tập hợp điểm
M
biểu diễn số phức
là.
A. Đường thẳng. B. Parabol. C. Đường tròn. D. Hai đường thẳng.
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y
O
P
M
Q
R
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u 36.(THPT chuyên Thái Bình) Trong mt phng tọa độ
Oxy
, tp hp các điểm biu diễn số
phức
z
thỏa mãn
(1 )zi
là s thc là.
A. Trc
Ox
. B. Đưng tròn bán kính bng
1
.
C. Đưng thng
yx
. D. Đưng thng
yx
.
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u 37.(THPT Hoàng Hoa Thám 2019) Trong mặt phẳng phức với hệ tọa độ
Oxy
, điểm biểu diễn
của các số phức
3z bi
với
b
luôn nằm trên đường có phương trnh là :
A.
3y
. B.
3yx
. C.
3x
. D.
yx
.
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u 38.(THPT Nhân Tông 2019) Tập hợp điểm biểu diễn số phức
thỏa mãn
23
1
4
zi
zi


A. Đường thẳng
3 1 0xy
. B. Đường thẳng
3 1 0xy
.
C. Đường tròn tâm
2;3I
bán kính
1
. D. Đường tròn tâm
4 ;1I
bán kính
1
.
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u 39(THPT Tiên Du 2017) Tập hợp các điểm trong mặt phẳng biểu diễn cho số phức
thoả
mãn điều kiện
2
2
zz
.
A. Gồm cả trục hoành và trục tung. B. Đường thẳng
yx
.
C. Trục hoành. D. Trục tung.
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u 40.(THPT Nguyễn Khuyến 2017) Tm tập hợp điểm biểu diễn số phức
thỏa mãn
23z i i z
.
A. Đường tròn có phương trnh
22
4xy
.
B. Elip có phương trnh
22
44xy
.
C. Đường thẳng có phương trnh
2 3 0xy
.
D. Đường thẳng có phương trnh
2 1 0xy
.
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u 41.(THPT Nguyễn Huệ-Huế) Trong mặt phẳng tọa độ
Oxy
, tm tập hợp điểm biểu diễn số
phức
thỏa mãn
1
zi
zi
.
A. Đường tròn
22
1 1 1xy
.
B. Hai đường thẳng
1y 
, trừ điểm
0; 1
.
C. Hnh ch󰊀 nhật giới hạn bởi các đường thẳng
1x 
;
1y 
.
D. Trục
Ox
.
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u 42.(TTLT ĐH Diệu Hiền 2017) Cho số phức
z
thỏa:
2 2 3 2 1 2z i i z
. Tập hợp điểm
biểu diễn cho số phức
z
là.
A. Một đường thng có phương trnh:
20 16 47 0xy
.
B. Một đường có phương trnh:
2
3 20 2 20 0y x y
.
C. Một đường thng có phương trnh:
20 16 47 0xy
.
D. Một đường thng có phương trnh:
20 32 47 0xy
.
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u 43.(THPT Hai Trưng- Huế) Tm tập hợp nh󰊀ng điểm
M
biểu diễn số phức
trong mặt
phẳng phức, biết số phức
thỏa mãn điều kiện
21z i z
.
A. Tập hợp nh󰊀ng điểm
M
là đường thẳng có phương trnh
2 4 3 0xy
.
B. Tập hợp nh󰊀ng điểm
M
là đường thẳng có phương trnh
4 2 3 0xy
.
C. Tập hợp nh󰊀ng điểm
M
là đường thẳng có phương trnh
2 4 3 0xy
.
D. Tập hợp nh󰊀ng điểm
M
là đường thẳng có phương trnh
4 2 3 0xy
.
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u 44.(Cụm 8 Hồ Chí Minh 2017) Trong mặt phẳng phức, tập hợp các điểm biểu diễn của số
phức
thỏa mãn điều kiện
2z i z
là đường thẳng
có phương trnh.
A.
2 4 13 0xy
. B.
4 2 3 0xy
. C.
4 2 3 0xy
. D.
2 4 13 0xy
.
Li gii
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u 45.(THPT Chuyên Bến Tre 2017) Cho số phức
thỏa mãn điều kiện
3 2 2 3z i z i
.
Tập hợp các điểm
M
biểu diễn cho
là đường thẳng có phương trnh.
A.
1yx
. B.
1yx
. C.
1yx
. D.
yx
.
Li gii
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u 46.(THPT Gia Lộc 2019) Cho số phức
3z m m i
,
m
. Tìm
m
để điểm biểu diễn của
số phức
z
nằm trên đường phân giác của góc phần tư thứ hai và thứ tư.
A.
3
2
m
. B.
2
3
m
. C.
1
2
m
. D.
0m
.
Li gii
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u 47.(THPT Chuyên Khiết 2019) Cho số phức
thỏa mãn
2 2 3 2 1 2z i i z
. Tập hợp
các điểm
M
biểu diễn số phức
trong mặt phẳng tọa độ
Oxy
đường thẳng phương trnh
nào sau đây:
A.
20 16 47 0xy
. B.
20 16 47 0xy
. C.
20 16 47 0xy
. D.
20 16 47 0xy
.
Lời giải
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u 48.(Sở GD & ĐT Tĩnh 2019) Tập hợp điểm biểu diễn số phức
thỏa mãn
2
2
zz
là.
A. một đường tròn. B. một điểm. C. một đường thẳng. D. một đoạn thẳng.
Lời giải
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u 49.(THPT Hoàng Văn Thụ 2019) Cho các số phức
thỏa mãn
1 1 2z i z i
. Tập hợp
các điểm biểu diễn các số phức
trên mặt phẳng tọa độ một đường thẳng. Viết phương trnh
đường thẳng đó.
A.
4 6 3 0xy
. B.
4 6 3 0xy
. C.
4 6 3 0xy
. D.
4 6 3 0xy
.
Lời giải
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u 50.(Sở GD&ĐT Bình Phước 2019) Trong mặt phẳng phức tập hợp điểm
Mz
thoả mãn
10
oo
z z z z
với
1
o
zi
là đường thẳng có phương trnh.
A.
2 2 1 0xy
. B.
2 2 1 0xy
. C.
2 2 1 0xy
. D.
2 2 1 0xy
.
Lời giải
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u 51. Tập hợp các điểm biểu diễn số phức
thỏa mãn
3 2 3z z i z
là:
A. Là một phần của đường thẳng
3yx
. B. Là một phần của đường thẳng
3yx
.
C. Là một phần của đường thẳng
3yx
. D. Là một phần của đường thẳng
3yx
.
Lời giải
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u 52. Cho số phức
12w i z
biết
12iz z i
. Khẳng định nào sau đây khẳng định
đúng?
A. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường elip.
B. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là 2 điểm.
C. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường thẳng.
D. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường tròn.
Lời giải
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u 53. Trong mặt phẳng phức, tập hợp các điểm
M
biểu diễn số phức
z
thỏa mãn điều kiện
34z z i
là?
A. Đường thẳng
6 8 25 0xy
. B. Đường tròn
22
40xy
.
C. Elip
22
1
42
xy

. D. Parabol
2
4yx
.
Lời giải
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u 54.(THPT Nguyễn Ti Học 2017) Cho số phức
12w i z
biết
12iz z i
.
Khẳng định nào sau đây là khẳng định đúng ?
A. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường thẳng.
B. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là 2 điểm.
C. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường elip.
D. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường tròn.
Lời giải
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u 55.(S GD & ĐT Đồng Nai 2019) Trong mặt phẳng với hệ trục tọa độ
Oxy
, tm tập hợp
T
các
điểm biểu diễn của các số phức
thỏa
10z
và phần ảo của
bằng
.
A.
6;8 , 6; 8T 
. B.
8;6 , 8;6T 
.
C.
T
là đường tròn tâm
O
bán kính
6R
. D.
T
là đường tròn tâm
O
bán kính
10R
.
Lời giải
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u 56.(THPT Thanh Thủy 2019) Cho số phức
;,z a bi a b
.
Để điểm biểu diễn của
nằm trong dải
2;2
(Hnh vẽ) điều
kiện của
,
là.
A.
2 2;ab
. B.
, 2;2ab
.
C.
2
2
a
b


. D.
2
2
a
b
.
Lời giải
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u 57.(TTGDTX Cam Lâm-Khánh Hòa) Cho số phức
,z x yi x y
. Tập hợp các điểm biểu
diễn của
sao cho
zi
zi
một số thực âm là?
A. Các điểm trên trục tung với
11y
. B. Các điểm trên trục tung với
1
1
y
y

.
C. Các điểm trên trục tung với
11y
. D. Các điểm trên trục hoành với
11x
.
Lời giải
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Bài toán 2. Tập hợp là một đường một đường tròn
2 2 2
22
( ) ( )
2 2 0
x a y b R
x y ax by c
1. Nhận dạng trắc nghiệm.
Khi gp gi thiết s phc có dng
z a bi R
1.
Ta nghĩ ngay tập hợp biễu diễn của
số phức
là điểm
;M x y
nằm trên đường tròn
22
2
x a y b R
có tâm tâm
;I a b
, bán
kính
R
hoặc
22
.R a b c
Đặc biệt:
Nếu
22
2
x a y b R
hoặc
z a bi R
thì tập hợp biễu diễn hình tròn tâm
;I a b
, bán kính
R
.
Nếu
22
22
r x a y b R
hoặc
r z a bi R
thì tập hợp biễu diễnhình vành
khăn giới hạn bởi hai đường tròn đồn tâm
;I a b
, bán kính lần lượt là
,rR
.
2. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
.,z x y i x y
c 2. Biến đổi điều kin
1
để tìm mi liên ca
x
y
giống như các dng trên.
c 3. Kết lun.
3. Kiến thức bỗ trợ.
Để viết phương trnh đường tròn ta cần tm tâm
( ; )I a b
và bán kính
.R
O
y
x
2
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Loại 1:
2 2 2
T
( ): .:
âm
(
BK:
;
(
(
)(
)
))C
R
C x a y b R
I a b
Loại 2:
22
,( ): 2 2 0C x y ax by c
với
22
0.R a b c
Chu vi đường tròn
()
2
C
pR
và diện tích đường tròn
2
()
.
C
SR
4. Bài tập minh họa.
Bài tp 8. Trong mặt phẳng tọa độ
,Oxy
tm tập hợp nh󰊀ng điểm biểu diễn số phức
thỏa
mãn điều kiện
(3 4 ) 2.zi
Li gii.
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Bài tp 9. Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
thỏa mãn điều
kin:
1 2 1zi
Li gii.
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Bài tp 10. Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
tha mãn
điu kin:
1z i i z
Li gii.
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Bài tp 11. Tìm tp hợp điểm
M
biu din s phc z tha
2
2
zi
zi
có phần thực bằng
3
.
Li gii
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Bài tp 12. Trong mt phng phc, tìm tp hợp các điểm biu din ca các s phc
a).
2 3 5 2zi
b).
5 4 3 1iz
Li gii
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Bài tp 13. Hãy xác định tập hợp các điểm trong mặt phẳng phức biểu diễn các số phức
thỏa
mãn
1 1 2.z
Li gii
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5. Câu hỏi trắc nghiệm.
Mc độ 2. Thông hiu
u 58. Nếu tp hợp các điểm biu din s phc
z
trong mt phng tọa độ
Oxy
một đường
tròn có phương trnh
22
9xy
thì
A.
1
3
z
. B.
3z
. C.
9z
. D.
1
9
z
.
Li gii
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u 59.(Đặng Thành Nam 2019) Tập hợp tất cả các điểm biểu diễn số phức
,z x yi x y
thỏa mãn
4zi
là đường cong có phương trnh
A.
22
( 1) 4.xy
B.
22
( 1) 4.xy
C.
22
( 1) 16xy
D.
22
( 1) 16.xy
Li gii
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u 60.(THPT Nguyễn Văn Cừ 2019) Cho số phức
z
thỏa
12zi
. Chọn mệnh đề đúng trong
các mệnh đề sau.
A. Tập hợp điểm biểu diễn số phức
z
là một đường thẳng.
B. Tập hợp điểm biểu diễn số phức
là một đường tròn có bán kính bằng
2
.
C. Tập hợp điểm biểu diễn số phức
là một đường Parabol.
D. Tập hợp điểm biểu diễn số phức
là một đường tròn có bán kính bằng
4
.
Li gii
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u 61.(Cho số phức
z
có tập hợp điểm biểu diễn trên mặt phẳng phức đường tròn có phương
trình
22
25 0xy
. Tính môđun của số phức
?
A.
3z
. B.
25z
. C.
5z
.
D.
2z
.
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u 62.(THPT Chuyên Phan Bội Châu 2017) Tm tập hợp điểm biểu diễn số phức
z
thỏa mãn
23zi
.
A. Đường tròn tâm
2;1I
, bán kính
3R
. B. Đường tròn tâm
2;1I
, bán kính
3R
.
C. Đường tròn tâm
1; 2I
, bán kính
3R
.
D. Đường tròn tâm
2; 1I
, bán kính
1R
.
Li gii
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u 63. Tập hợp điểm biểu diễn các số phức thỏa
11zi 
một đường tròn. Tm tâm
của
đường tròn đó.
A.
1;0I
. B.
1;0I
. C.
0; 1I
. D.
0;1I
.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 64.(THPT Thái Phiên 2019) Cho số phức
thỏa mãn
32iz i
. Trong mặt phẳng
phức, quỹ tích điểm biểu diễn số phức
là hình vẽ nào dưới đây?
A.
B.
C.
D.
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u 65.(THPT Chuyên Hồng Phong 2019) Trong mặt phẳng phức, tập hợp các điểm
M
biểu
diễn số phức
z
thỏa mãn
2
1z z i
là một hnh
H
chứa điểm nào trong số bốn điểm sau?
A.
1
0; 1M
. B.
2
31
;
22
M




. C.
3
1;1M
. D.
4
13
;
22
M




.
Li gii
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u 66.(THPT Chuyên Bến Tre 2017) Cho số phức
thỏa mãn điều kiện:
3 2 3 2 3 z i z i
.
Tập hợp các điểm
M
biểu diễn cho số phức
là đường có phương trnh.
A.
22
15 25 9
8 8 32
xy
. B.
22
15 25 9
8 8 32
xy
.
C.
22
15 25 9
8 8 32
xy
. D.
22
15 25 9
8 8 32
xy
.
Li gii
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x
y
3
3
2
1
2
O
1
x
y
3
2
1
2
O
1
x
y
3
3
2
1
2
O
1
x
y
3
3
2
1
2
O
1
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 67.(Cụm Hồ Chí Minh) Cho số phức
z a bi
, với
a
b
hai số thực. Để điểm biểu diễn của
z
trong mặt phẳng tọa độ
Oxy
nằm hẳn bên trong hnh tròn tâm
O
bán kính
2R
như
hình bên th điều kiện cần và đủ của
a
b
là.
A.
22
4ab
. B.
22
2ab
. C.
2ab
. D.
4ab
.
Li gii
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u 68.(THPT n Thịnh 2017) Trong mp tọa độ
Oxy
, tm tập hợp điểm biểu diễn các số phức
thỏa mãn:
1z i i z
.
A. Tập hợp các điểm biểu diễn các số phức
là đường tròn tâm
0;1I
, bán kính R=
3
.
B. Tập hợp các điểm biểu diễn các số phức
là đường tròn tâm
2; 1I
, bán kính R=
2
.
C. Tập hợp các điểm biểu diễn các số phức
là đường tròn tâm
0; 1I
, bán kính R=
3
.
D. Tập hợp các điểm biểu diễn các số phức
là đường tròn tâm
0; 1I
, bán kính R=
2
.
Li gii
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u 69.(THPT Lương Tài 2018) Tập hợp các điểm biểu diễn số phức
thỏa mãn
(3 4 ) 2zi
trong mặt phẳng
Oxy
.
A. Đường tròn
22
6 8 21 0x y x y
. B. Đường thẳng
2 1 0xy
.
C. Parabol
2
23y x x
. D. Đường tròn
22
3 4 4xy
.
Li gii
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u 70.(TTGDTX Vạn Ninh 2019) Tập hợp các điểm trong mặt phẳng biểu diễn cho số
z
phức
thoả mãn điều kiện
1 2 4zi
là:
A. Một đoạn thẳng. B. Một đường thẳng. C. Một hnh vuông. D. Một đường tròn.
Li gii
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u 71.(THPT Thanh Thủy 2019) Trong mặt phẳng tọa độ
Oxy
, tập hợp nh󰊀ng điểm biểu diễn
số phức
thỏa mãn
1z i i z
.
A. Đường tròn có phương trnh
2
2
12xy
.
B. Hai đường thẳng có phương trnh
1, 2xx
.
C. Đường thẳng có phương trnh
10xy
.
D. Đường tròn có phương trnh
2
2
12xy
.
Li gii
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u 72.(THPT TH Cao Nguyên 2019) Trên mặt phẳng tọa độ, tập hợp điểm biểu diễn số phức
z
thỏa mãn điều kiện
2 5 6zi
là đường tròn có tâm và bán kính lần lượt là:
A.
(2; 5), 6IR
. B.
( 2;5), 36IR
. C.
(2; 5), 36IR
. D.
( 2;5), 6IR
.
Li gii
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u 73. Tm tập hợp các điểm biểu diễn của số phức
z
thỏa mãn
22zi i
.
A.
2 1 0xy
. B.
3 4 2 0xy
. C.
22
1 2 4xy
. D.
22
1 2 4xy
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 74. Tập hợp điểm biểu diễn các số phức thỏa
11zi 
một đường tròn. Tm tâm
của
đường tròn đó.
A.
1;0I
. B.
1;0I
. C.
0; 1I
. D.
0;1I
.
Li gii
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u 75. Tập hợp điểm biểu diễn số phức
z
thỏa điều kiện
1 2 1zi
nằm trên đường tròn
tâm là:
A.
1; 2I 
. B.
1; 2I
. C.
1;2I
. D.
1;2I
.
Li gii
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u 76.(THPT Trn P2018) Tập hợp các điểm biểu diễn số phức
z
thỏa mãn
3 2 10zi
là.
A. Đường tròn
22
2 3 100xy
. B. Đường thẳng
2 3 100xy
.
C. Đường thẳng
3 2 100xy
. D. Đường tròn
22
3 2 100xy
.
Li gii
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u 77. Trong mặt phẳng
Oxy
. Cho tập hợp điểm biểu diễn các số phức
thỏa mãn điều kiện
2 1 5iz
. Phát biểu nào sai?
A. Tập hợp điểm biểu diễn số phức
z
là đường tròn tâm
1; 2I
.
B. Tập hợp điểm biểu diễn số phức
là đường tròn có bán kính
5R
.
C. Tập hợp điểm biểu diễn số phức
là một hnh nón.
D. Tập hợp điểm biểu diễn số phức
là đường tròn có đường kính
10
.
Li gii
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u 78. Tm tập hợp điểm biểu diễn số phức
z
thỏa
22zi
.
A. Tập hợp điểm biểu diễn số phức
z
là đường tròn
22
4 2 1 0x y x y
.
B. Tập hợp điểm biểu diễn số phức
là đường tròn
22
4 2 1 0x y x y
.
C. Tập hợp điểm biểu diễn số phức
là đường tròn
22
4 2 4 0x y x y
.
D. Tập hợp điểm biểu diễn số phức
là đường tròn
22
4 2 4 0x y x y
.
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Li gii
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u 79.(Cụm 7-TP Hồ Chí Minh 2018) Tập hợp các điểm
M
biểu diễn của số phức
thoả mãn
2 5 4zi
là:
A. Đường tròn tâm
O
và bán kính bằng
.
B. Đường tròn tâm
2; 5I
và bán kính bằng
.
C. Đường tròn tâm
2; 5I
và bán kính bằng
.
D. Đường tròn tâm
2;5I
và bán kính bằng
.
Li gii
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u 80. Trong mặt phẳng tọa độ, tập hợp điểm biểu diễn số phức
thỏa mãn
1z i i z
một đường tròn, đường tròn đó có phương trnh là:
A.
22
2 2 1 0x y x y
. B.
22
2 1 0x y x
.
C.
22
2 1 0x y x
. D.
22
2 1 0.x y y
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u 81.(THPT chuyên Phan Bội Châu 2019) bao nhiêu số phức
thỏa mãn:
2zi
2
z
là số thuần ảo:
A.
2.
B.
4.
C.
3.
D.
1.
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u 82.(THPT chuyên Vĩnh Phúc 2019) Trong mặt phẳng
xOy
, gọi
M
điểm biểu diễn của số
phức
thỏa mãn
3 3 3zi
. Tm phần ảo của
trong trường hợp góc
xOM
nhỏ nhất.
A.
3
. B.
33
2
. C.
. D.
23
.
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u 83.(Sở GD & ĐT m Đồng 2018) Tập hợp các điểm trong mt phẳng phức biểu diễn các số
thỏa mãn điều kin:
1z i i z
là đường tròn có bán kính là.
A.
2R
. B.
2R
.
C.
4R
.
D.
1R
.
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u 84.(THPT Chuyên Nguyễn Bĩnh Khiêm 2019) Cho số phức
thỏa mãn
12z z i
một
số thuần ảo. Tập hợp điểm biểu diễn số phức
là một đường tròn có diện tích bằng.
A.
5
. B.
25
. C.
5
2
. D.
5
4
.
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u 85. Tm tập hợp các điểm
M
biểu diễn số phức
,z x yi x y
thỏa mãn
2
2
zi
zi
.
A. Đường tròn tâm
0;2I
bán kính
2R
. B. Đường tròn tâm
0; 2I
bán kính
2R
.
C. Đường tròn tâm
2;0I
bán kính
2R
. D. Đường tròn tâm
2;0I
bán kính
2R
.
Li gii
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u 86.(THPT Lệ Thủy 2019) Gọi
M
điểm biểu diễn của số phức
z
thỏa
1 3 4z m i
.
Tm tất cả các số thực
m
sao cho tập hợp các điểm
M
là đường tròn tiếp xúc với trục
Oy
.
A.
5; 3mm
. B.
5; 3mm
. C.
3m 
. D.
5m
.
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u 87.(THPT Chun Lê Hồng Phong 2019) Trên mặt phẳng phức, tập hợp các điểm biểu diễn số
phức
thỏa mãn
2z i z i
là một đường tròn có bán kính là
R
. Tính giá trị của
R
.
A.
1R
. B.
1
9
R
. C.
2
3
R
. D.
1
3
R
.
Li gii
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u 88.(Sở GD & ĐT Tĩnh 2019) Biết số phức
thõa mãn
11z 
zz
phần ảo không âm. Phần mặt phẳng biểu
diễn số phức
có diện tích là:
A.
. B.
2
. C.
2
. D.
2
.
Li gii
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x
y
O
-2
-1
-1
1
2
2
-2
1
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u 89.(PTNK-ĐHQG TP HCM 2018) Gọi
H
là tập hợp các điểm
biểu diễn số phức
thỏa
1 1 2z
trong mặt phẳng phức.
Tính diện tích hnh
H
.
A.
2
. B.
3
. C.
4
. D.
5
.
Li gii
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Bài toán 3. Tập hợp là một đường một đường Parabol
2
2
0
y ax bx c
c
x ay by c
1. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
.,z x y i x y
c 2. Biến đổi điều kin
1
để tìm mi liên ca
x
y
giống như các dng trên.
c 3. Kết lun là một parabol
()P
có đỉnh
;
24
b
I
aa



2. Bài tập minh họa.
Bài tp 14. Trong mt phng phc, tìm tp hợp các đim biu din ca s phc
tha mãn
điu kin:
22z i z z i
Li gii
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3. Câu hỏi trắc nghiệm.
Mc độ. Nhn biết
u 90.(THPT Nguyn Trãi 2019) Tp hợp các điểm trong mt phng tọa độ biu din s phc
thỏa mãn điều kin
22z i z z i
là hình gì?
A. Một đường thng. B. Một đường tròn. C. Một đường Parabol. D. Một đường Elip.
Li gii
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u 91.(THPT Lý Thái Tổ 2019) Tập hợp các điểm trong mặt phẳng biểu diễn cho số phức z thoả
mãn điều kiện
22z i z z i
là.
A. Một parabol. B. Một đường tròn. C. Một đường thẳng. D. Một elip.
Li gii
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u 92.(THPT Nguyễn Thái Học 2019) Cho số phức
2
z a a i
, với
a
. Khi đó điểm biểu diễn
của số phức
nằm trên :
A. Đường thẳng
1yx
. B. Parabol
2
yx
. C. Đường thẳng
2yx
. D. Parabol
2
yx
.
Li gii
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u 93.(THPT Chuyên Đại Học Vinh 2019) Gọi
M
là điểm biểu diễn của số phức
thỏa mãn
3 2 3z i z z i
. Tìm tập hợp tất cả những điểm
M
như vậy.
A. Một parabol. B. Một elip. C. Một đường tròn. D. Một đường thẳng.
Li gii
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u 94.(THPT Chuyên Qúy Đôn 2019) Trong mt phng
Oxy
, tp hp những điểm
;M x y
vi
,xy
biu din các s phc
z x yi
tha mãn
2 1 2z z z
đường phương
trình nào sau đây?
A.
2
4yx
. B.
2
4yx
. C.
2
2yx
. D.
2
2yx
.
Li gii
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u 95.(THPT Kim Liên 2019) Cho s phc
2
36z m m m i
vi
m
. Gi
P
là tp hp
các điểm biu din s phc
z
trong mt phng tọa độ. Din tích hình phng gii hn bi
P
trc hoành bng
A.
125
6
. B.
17
6
. C. 1. D.
55
6
.
Li gii
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u 96.(THPT Đô Lương 2019) Xét các s phc
tha mãn
1
1
zi
z z i


s thc. Tp hp các
đim biu din ca s phc
w
2
z
là parabol có đỉnh
A.
13
;
44
I



. B.
11
;
22
I



. C.
13
;
22
I



. D.
11
;
44
I



.
Li gii
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u 97.(THPT Kim Liên 2019) Cho s phc tha mãn
12z i z i
. Tp hợp điểm biu din s
phc
21w i z
trên mt phng phc là một đường thẳng. Phương trình của đường thng
A.
7 9 0xy
B.
7 9 0xy
C.
7 9 0xy
D.
7 9 0xy
Li gii
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u 98.(THPT Chuyên Sơn La Lần 2019) Xét các số phức
z
thỏa mãn điều kiện
1z i z i
số thực. Biết rằng tập hợp các điểm biểu diễn hình học của
z
một đường thẳng. Hệ số góc của
đường thẳng đó là
A.
1
. B.
1
. C.
2
. D.
2
.
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u 99.ng Thành Nam 2019) Cho s phc
3
( ) ,z m m m i
vi
m
tham s thực thay đổi.
Tập hơp tất c các điểm biu din s phc
z
đường cong
()C
.Tính din tích hình phng gii
hn bi
()C
và trc hoành.
A.
1
2
. B.
1
4
. C.
3
4
. D.
3
2
.
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Li gii
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u 100.(Tạp Chí Tn Học Số 3 tháng 2018) Tập hợp các điểm biểu diễn số phức
z
thỏa mãn
2 1 2 z z z
trên mặt phẳng tọa độ là một
A. đường thẳng. B. đường tròn. C. parabol. D. hypebol.
Li gii
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u 101.(S GD & ĐT Phú Th 2019) Cho s phc
z
tha mãn
3 1 3z i z i
mt s thc.
Biết rng tp hợp các điểm biu din ca
z
một đường thng. Khong cách t gc tọa độ đến
đưng thẳng đó bằng
A.
42
. B.
0
. C.
22
. D.
32
.
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u 102.(THPT Chuyên Hùng Vương 2019) Cho s phc
thỏa điểu kin
22z i z z i
.
Khẳng định nào sau đây đúng?
A. Tp hợp các điểm biu din s phc
trong mt phng phức là 1 đường parabol.
B. Tp hợp các điểm biu din s phc
trong mt phng phức là 1 đường thng.
C. Tp hợp các điểm biu din s phc
trong mt phng phức là 1 đường hypebol.
D. Tp hợp các điểm biu din s phc
trong mt phng phức là 1 đường tròn.
Li gii
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Bài toán 4. Tập hợp là một đường một đường Elíp
:E
22
22
1.
xy
ab

1. Nhận dạng trắc nghiệm.
Khi gp gi thiết s phc có dng
1 1 2 2
2z a bi z a b i a
1.
Ta nghĩ ngay tập hợp biểu diễn của số phức
là điểm
;M x y
nằm trên:
Đoạn thẳng
AB
nếu
2a AB
với
1 1 2 2
; , ; .A a b B a b
Elíp nếu
2a AB
với
1 1 2 2
; , ; .A a b B a b
Khi đó
E
nhận
1 1 2 2
; , ;A a b B a b
làm hai tiêu điểm và độ dài trục lớn là
2.a
2. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
.,z x y i x y
c 2. Biến đổi điều kin
1
để tìm mi liên ca
y
giống như các dng trên.
c 3. Kết lun.
3. Kiến thức bỗ tr.
Định nghĩa: Cho hai điểm cố định
1
F
2
F
với
12
2 0.F F c
Đường elip tập hợp các điểm
M
sao cho
12
2 , ( ).MF MF a a c
Hai điểm
12
, FF
gọi là các tiêu điểm của elip.
Khoảng cách
2c
được gọi là tiêu cự của elip.
Phương trình chính tắc của elip:
22
22
( ) : 1
xy
E
ab

với
0.ab
c thông số cần nhớ:
Trục lớn
12
2.A A a
Trục bé
12
2.B B b
Tiêu cự
12
2.F F c
Mối liên hệ
2 2 2
.a b c
Bán kính qua tiêu của
M
1 2 1 2
, 2 .
cc
MF a x MF a x MF MF a
aa
4. Bài tập minh họa.
Bài tp 15. Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
tha mãn
điu kin:
2 2 5.zz
Li gii.
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Bài tp 16. Biết tập hợp các điểm
M
biểu diễn hình học số phức
thỏa
4 4 10zz
một elip
( ).E
Hãy viết phương trình elip đó.
Li gii.
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Bài tp 17. Biết tập hợp các điểm
M
biểu diễn hình học số phức
thỏa
4z i z i
một
elip
( ).E
Hãy viết phương trình elip đó.
Li gii
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Bài tp 18. Trong mt phng phc, tp hợp các điểm biu din ca s phc
4 3 3 2 10.z i z i
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5. Câu hỏi trắc nghiệm.
Mc độ 1. Nhn biết
u 103.(THPT Chuyên Nguyên Giáp 2019) Gọi
H
hình biểu diễn tập hợp các số phức
trong mặt phẳng tọa độ
0xy
sao cho
23zz
, và số phức
có phần ảo không âm. Tính diện tích
hình
H
.
A.
3
. B.
3
2
. C.
3
4
. D.
6
.
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u 104.(THPT Chuyên Bến Tre 2019) Cho số phức
thỏa mãn điều kiện:
4 4 10zz
. Tập
hợp các điểm
M
biểu diễn cho số phức
là đường có phương trình.
A.
22
1
9 25
xy

. B.
22
1
25 9
xy

. C.
22
1
9 25
xy

. D.
22
1
25 9
xy

.
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u 105.(THPT Kim Liên 2019) Cho số phức
thỏa mãn
4 4 10.zz
Trong các khẳng định
sau khẳng định nào đúng?
A. Tập hợp điểm biểu diễn số phức
z
là một parabol.
B. Tập hợp điểm biểu diễn số phức
là một đường tròn.
C. Tập hợp điểm biểu diễn số phức
là một elip.
D. Tập hợp điểm biểu diễn số phức
là một đường thẳng.
Li gii
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u 106.(Sở GD & ĐT Bình Phước 2019) Cho số phức
thỏa mãn
2 2 8zz
. Trong mặt
phẳng phức tập hợp những điểm
M
biểu diễn cho số phức
z
là?
A.
22
: 2 2 64C x y
. B.
22
:1
16 12
xy
E 
.
C.
22
:1
12 16
xy
E 
. D.
22
: 2 2 8C x y
.
Li gii
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u 107.(THPT HaiTrưng-Huế) Tìm tập hợp các điểm
M
biểu diễn hình học số phức
z
trong
mặt phẳng phức, biết số phức
thỏa mãn điều kiện:
4 4 10.zz
.
A. Tập hợp các điểm cần tìm là đường elip có phương trình
22
1
9 25
xy

.
B. Tập hợp các điểm cần tìm là những điểm
;M x y
trong mặt phẳng
Oxy
thỏa mãn phương
trình
22
22
4 4 12x y x y
.
C. Tập hợp các điểm cần tìm là đường tròn có tâm
0;0O
và có bán kính
4R
.
D. Tập hợp các điểm cần tìm là đường elip có phương trình
22
1
25 9
xy

.
Li gii
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u 108.(THPT Lương Thế Vinh 2019) Cho số phức
z
thỏa mãn
2 2 4zz
. Tập hợp điểm
biểu diễn của số phức
z
trên mặt phẳng tọa độ là
A. Một đường elip. B. Một đường parabol. C. Một đoạn thẳng. D. Một đường tròn.
Li gii
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u 109.(S GD & ĐT Ninh Bình 2019) Hình phng gii hn bi tp hợp điểm biu din các s
phc
tha mãn
3 3 10zz
có din tích bng
A.
12
. B.
20
. C.
15
. D.
25
.
Li gii
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u 110.(Toán Hc Tui Tr 2019) Trong mt phng
Oxy
, gi (
H
) hình biu din tp hp các
s phc z tha mãn
7 10zz
. Din tích ca hình (
H
) bng
A.
5
2
. B.
25
12
. C.
7
2
. D.
5
.
Li gii
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u 111.(Chuyên KHTT 2019) Cho s phc
thay đổi tha mãn
6z i z i
. Gi
S
đường
cong to bi tt c các điểm biu din s phc
1z i i
khi
z
thay đổi. Tính din tích hình
phng gii hn bởi đường cong
S
.
A.
12
. B.
12 2
. C.
92
. D.
BF
.
Trung Tâm Luyện Thi Đại Học Amsterdam Chương IVBài 2. Tập Hợp Điểm-Cực Tr
113
Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
Li gii
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Bài toán 5. Tập hợp biểu diễn của số phức
1w f z
thỏa mãn điều kiện
2
của số phức
z
.
1. Phương pháp.
S dụng phương pháp rút thế: tc là t
1
:
w f z
rút
theo hàm số theo biến
w
.
Sau đó thay vào điều kiện
2
ri biến đổi tìm mi liên ca
y
giống như các dng trên.
Kết lun.
Đặt bit: nếu điều kin
2
cho
za
hoặc
z b a
thì ta sử dụng kỹ thuật lấy môđun hai vế.
2. Bài tập minh họa.
Bài tp 19. Cho các số phức
thỏa mãn
1 2 .z i z i
Tập hợp các điểm biểu diễn số phức
(2 ) 1w i z
trên mặt phẳng tọa độ là một đường thẳng. Viết phương trình đường thẳng đó.
Li gii
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Bài tp 20. Trong mt phng phc, tìm tp hợp các điểm biu din ca s phc
1 3 2w i z
, trong đó
là s phc tha mãn
12z 
.
Li gii
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Bài tp 21. Cho số phức
thỏa mãn
2 5.z 
Biết rằng tập hợp điểm biểu diễn của số phức
(1 2 ) 3w i z
là một đường tròn tâm
và bán kính
.R
Tìm
.R
Trung Tâm Luyện Thi Đại Học Amsterdam Chương IVBài 2. Tập Hợp Điểm-Cực Tr
114
Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
Li gii
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Bài tp 22. Cho số phức
thỏa
10
(2 ) 1 2 .i z i
z
Biết tập hợp điểm biểu diễn của số phức
(3 4 ) 1 2w i z i
là một đường tròn tâm
I
và bán kính
.R
Tìm
I
.R
Li gii
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4. Câu hỏi trắc nghiệm.
Mc độ 3. Vn dng
u 112.(Cụm 6-Hồ Chí Minh 2017) Cho số phức
4z
. Tập hợp các điểm
M
trong mặt
phẳng tọa độ
Oxy
biểu diễn số phức
3w z i
là một đường tròn. Tính bán kính đường tròn đó.
A.
4
3
. B.
4
. C.
42
. D.
3
.
Lời giải
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u 113.(THPT Chuyên Thái Nguyên 2018) Cho số phức
z
thỏa mãn điều kiện
3.z
Biết rằng
tập hợp tất cả các điểm biểu diễn số phức
3 2 2w i i z
một đường tròn. Hãy tính bán
kính của đường tròn đó.
A.
33
. B.
32
. C.
37
.
D.
35
.
Li gii
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115
Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 114.(TTGDTX Vạn Ninh 2019) Cho các số phức
z
thỏa mãn
2z
. Biết rằng tập hợp các điểm
biểu diễn các số phức
3 2 2w i i z
là một đường tròn. Tính bán kính
r
của đường tròn đó.
A.
6r
. B.
6r
. C.
20r
. D.
20r
.
Li gii
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u 115.(THPT Gia Lc 2019) Cho s phc
z
tha mãn
2z
. Biết rng tp hợp các điểm biu diễn
s phc
w 3 2 2i i z
là một đường tròn. Tìm tọa độ tâm
I
của đường tròn đó?
A.
3; 2I
. B.
3;2I
. C.
3;2I
. D.
3; 2I 
.
Li gii
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u 116.(Sở GD & ĐT Quảng Nam 2019) Cho số phức
z
đun bằng
22
. Biết rằng tập hợp
điểm trong mặt phẳng tọa độ biểu diễn các số phức
w = 1 1i z i
đường tròn tâm
;I a b
bán kính
R
. Tổng
a b R
bằng:
A.
5
. B.
. C.
1
. D.
3
.
Li gii
Trung Tâm Luyện Thi Đại Học Amsterdam Chương IVBài 2. Tập Hợp Điểm-Cực Tr
116
Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 117.(SGD & ĐT Nam Định 2019) Cho số phức
thỏa mãn
3.z
Biết rằng tập hợp điểm
biểu diễn số phức
w z i
là một đường tròn. Tìm tọa độ tâm của đường tròn đó.
A.
0;1
. B.
0; 1
. C.
1;0
. D.
1;0
.
Li gii
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u 118. Cho các số phức z thỏa mãn
4z
. Biết rằng tập hợp các điểm biểu diễn các số phức
34w i z i
là một đường tròn. Tính bán kính
r
của đường tròn đó.
A.
20r
. B.
4r
. C.
5r
. D.
22r
.
Li gii
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u 119.(Sở GDĐT Lâm Đồng 2017) Cho các số phức
thỏa mãn
12z
. Biết rằng tập hợp
các điểm biểu diễn các số phức
(1 3) 2 w i z
một đường tròn. Bán kính
r
của đường
tròn đó
A.
2r
. B.
4r
. C.
8r
. D.
16r
.
Li gii
Trung Tâm Luyện Thi Đại Học Amsterdam Chương IVBài 2. Tập Hợp Điểm-Cực Tr
117
Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 120.(Sở GDĐT Lâm Đồng 2017) Cho số phức
thỏa mãn
12z 
. Biết rằng tập hợp các
điểm biểu diễn các số phức
2w z i
là một đường tròn. Tìm bán kính
r
của đường tròn đó.
A.
2r 
. B.
1r
. C.
2r
. D.
4r
.
Li gii
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u 121.(TTGDTX Cam Lâm 2018) Cho số phức
z
thỏa mãn
3 4 2zi
2 1-w z i
. Trong
mặt phẳng phức, tập hợp điểm biểu diễn số phức
w
là đường tròn tâm
I
, bán kính
R
. Khi đó:
A.
( 7;9), 16IR
. B.
( 7;9), 4IR
. C.
(7; 9), 16IR
. D.
(7; 9), 4IR
.
Li gii
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u 122.(Cụm 6 Hồ Chí Minh 2017) Cho số phức
thỏa mãn
1 2; (1 3 ) 2z w i z
. Tập hợp
điểm biểu diễn của số phức
w
là đường tròn, tính bán kính đường tròn đó.
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
A.
3R
. B.
4R
. C.
5R
. D.
2R
.
Lời giải
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u 123. Cho số phức
1 3 2w i z
biết rằng
12z 
. Khi đó khẳng định nào sau đây
khẳng định đúng.
A. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một parabol.
B. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường tròn.
C. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một elip.
D. Tập hợp điểm biểu diễn số phức
w
trên mặt phẳng phức là một đường thẳng.
Lời giải
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u 124.(THPT Trần P2017) Cho số phức
z
thỏa mãn
3 4 2zi
21w z i
. Trong mặt
phẳng phức, tập hợp điểm biểu diễn số phức
w
là đường tròn tâm
I
, bán kính
R
là.
A.
7; 9 , 4IR
. B.
7; 9 , 16IR
. C.
7;9 , 4IR
. D.
7;9 , 16IR
.
Li gii
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u 125.(THPT Hồng Phong 2018) Cho các số phức
thỏa mãn
12z 
. Biết rằng tập hợp
các điểm biểu diễn các số phức
1 3 2w i z
một đường tròn. Tính bán kính
r
của đường
tròn
A.
25r
. B.
4r
. C.
9r
. D.
16r
.
Li gii
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u 126.(Cụm 4 Hồ Chí Minh 2019) Cho số phức
z
thỏa mãn
22z 
. Biết rằng tập hợp các
điểm biểu diễn các số phức
1w i z i
là một đường tròn. Tính bán kính
r
của đường tròn đó
A.
4r
. B.
2r
. C.
22r
. D.
2r
.
Li gii
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u 127.(THPT Chuyên Thái Nguyên 2018) Tập hợp các số phức
11w i z
với
số phức
thỏa mãn
11z 
là hình tròn. Tính diện tích hình tròn đó.
A.
2
. B.
. C.
3
. D.
4
.
Li gii
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u 128.(THPT Chuyên Quang Trung 2019) Cho thỏa mãn
z
thỏa mãn
10
2 1 2i z i
z
.
Biết tập hợp các điểm biểu diễn cho số phức
3 4 1 2w i z i
đường tròn
, bán kính
R
.
Khi đó.
A.
1; 2 , 5IR
. B.
1;2 , 5IR
. C.
1;2 , 5IR
. D.
1; 2 , 5IR
.
Li gii
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u 129.(THPT Thuận Thành 2019) Cho số phức
. Biết tập hợp các điểm biểu diễn số phức
34w i z i
là một đường tròn có bán kính bằng
20
. Tính
z
.
A.
2z
. B.
10z
. C.
8z
. D.
4z
.
Li gii
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u 130.(THPT Chuyên Hunh Mẫn Đạt 2019) Cho s phc
tha mãn
1 2 2zi
. Tp hp
đim biu din s phc
1
z
w
i
trong mt phng to độ
Oxy
là đường tròn có tâm là
A.
13
;
22
I



. B.
13
;
22
I



. C.
31
;
22
I




. D.
31
;
22
I



.
Li gii
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u 131.(Đặng Thành Nam 2019) Cho số phức
thay đổi thỏa mãn
3 4 2zi
.
Đặt
2 2 2 1w z i
, tập hợp tất cả các điểm biểu diễn số phức
w
một hình tròn diện
tích bằng
A.
8
. B.
12
. C.
16
. D.
32
.
Li gii
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u 132.(Cm Trần Kim Hưng 2019) Trong mt phng tọa độ
Oxy
. Tìm tp hợp điểm
M
biu
din s phc
w 1 2 . 3iz
, biết
z
tha mãn
25z 
?
A.
22
1 2 125xy
. B.
2x
.
C.
22
5 4 125xy
. D.
22
1 4 125xy
.
Li gii
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u 133.(THPT TX Quãng Tr 2019) Cho s phc
z
tha mãn
2
2 4 6z i m m
, vi
m
s
thc. Biết rng tp hp các điểm biu din ca s phc
(4 3 ) 2w i z i
đường tròn. Bán kính
của đường tròn đó có giá trị nh nht bng
A.
10
. B. 2. C. 10. D.
2
.
Li gii.
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u 134.(THPT Ngô Quyn Hi Phòng 2019) Cho s phc
thay đổi tha mãn
1 2.z 
Biết rng
tp hp các s phc
1 3 2w i z
là đường tròn có bán kính bng
.R
Tính
.R
A.
8R
. B.
2R
. C.
16R
. D.
4R
.
Li gii
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u 135.(THPT Chuyên Thái Nguyên) Cho các số phức
z
thỏa mãn
12z 
. Biết rằng tập hợp
các điểm biểu diễn các số phức
18w i z i
một đường tròn. Bán kính
r
của đường tròn
đó là
A.
9
. B.
36
. C.
6
. D.
3
.
Li gii
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u 136.(THPT Kinh Môn-Hi Dương 2019) Cho s phc
z
tha mãn
1 3 3 2zi
. Biết rng
s phc
2019
1 3 2019w i z i
tp hợp các điểm biu din thuộc đường tròn
C
. Din
tích
S
ca hình tròn
C
bng
A.
18
. B.
36
. C.
9
. D.
12
.
Li gii
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u 137.(Chuyên Đại Hc Vinh) Gi tp hp tt c các s nguyên sao cho tn ti s
phc phân bit thỏa mãn đồng thời các phương trình . Tng
tt c các phn t ca
A. . B. . C. . D. .
Li gii
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Mc độ 4. Vn dng cao
1. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
.,z x y i x y
c 2. Biến đổi điều kin đã cho để tìm mi liên ca
y
.
c 3. S dụng các phép tương giao của đường thẳng, đường tròn… hoặc các tính cht tọa độ
trong hình hc phng
Oxy
như trung điểm, trọng tâm, độ dài…
2. Câu hỏi trắc nghiệm.
u 138.(THPT Chuyên Thái Nguyên 2019) Cho
12
,zz
hai số phức thỏa mãn
| 5 3 | 5zi
đồng
thời
12
| | 8zz
. Tập hợp các điểm biểu diễn số phức
12
w z z
trong mặt phẳng tọa độ
Oxy
đường tròn có phương trình
A.
22
( 10) ( 6) 36xy
. B.
22
( 10) ( 6) 16xy
.
C.
22
53
( ) ( ) 9
22
xy
. D.
22
5 3 9
( ) ( )
2 2 4
xy
.
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S
m
2
12
,zz
1z z i
21z m m
S
1
4
2
3
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u 139.(Chuyên Đại Hc Vinh) Gi tp hp tt c các s sao cho tn tại đúng một s
phc thỏa mãn đồng thời các phương trình . Tích
tt c các phn t ca
A. . B. . C. . D. .
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u 140.(Chuyên Đại Hc Vinh) Gi tp hp tt c các s nguyên sao cho tn ti s
phc phân bit thỏa mãn đồng thời các phương trình .
S các phn t ca
A. . B. . C. . D. .
Li gii
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S
m
z
21z i z
2
2 3 2 5 9z i m m
S
6
5
2
3
S
m
2
12
,zz
3 4 25 20iz
25z m i
S
8
7
6
5
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u 141.(Sở GD & ĐT Thanh Hóa 2019) Gọi
1
z
,
2
z
hai trong các số phức thỏa mãn
1 2 5zi
12
8zz
. Tìm mô đun của số phức
12
24w z z i
.
A.
6w
. B.
10w
. C.
16w
. D.
13w
.
Li gii
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u 142.ng Thành Nam) Cho s phc z tho mãn
11z 
zz
phn o không âm. Tp
hợp các điểm biu din s phc
z
là mt min phng. Tính din tích
S
ca min phng này
A.
S
. B.
2S
. C.
1
2
S
. D.
1S
.
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u 143.(Tp Chí Toán Hc 2019) Phn gch trong hình v i
là hình biu din ca tp các s phc thỏa mãn điều kin nào sau
đây?
A.
68z
. B.
2 4 4 4zi
.
C.
2 4 4 4zi
. D.
4 4 4 16zi
.
Li gii
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u 144.(Sở GDT Bình Thuận 2019) Gọi
12
,zz
là hai trong các số phức
z
thỏa mãn
12
6zz
3 5 5zi
. Tìm môđun của số phức
12
6 10z z i
.
A.
10
. B.
32
. C.
16
. D.
8
.
Li gii
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u 145. Biết tập hợp tất cả các điểm biểu diễn số phức
đường tròn cho bởi hình vẽ bên. Hỏi tập hợp tất cả các
điểm biểu diễn số phức
34zi
được thể hiện bởi đường
tròn trong hình vẽ nào trong bốn hình vẽ dưới đây?
A.
B.
C.
D.
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x
y
2
-4
-3
-3
3
3
-2
-2
2
-1
-1
1
1
O
2
x
y
-4
-3
-3
3
-2
-2
2
-1
-1
1
1
O
2
x
y
-4
-3
-3
3
-2
-2
2
-1
-1
1
1
O
2
x
y
-4
-3
-3
3
-2
-2
2
-1
-1
1
1
O
2
x
y
-4
-3
-3
3
-2
-2
2
-1
-1
1
1
O
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u 146. (THPT Chuyên Quc Hc Huế) Cho s phc
z
tha mãn
2
3 i . 9z z z
. Tìm tp hp
đim biu din s phc
tha mãn
1iz
A. Hình tròn
2
2
5 73
1
8 64
xy



. B. Đưng tròn
2
2
5 73
1
8 64
xy



.
C. Đưng tròn
22
1 3 9xy
. D. Hình tròn
22
1 3 9xy
.
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u 147.(Chuyên phạm Ni 2019) Trong mt phng tọa độ
Oxy
,
gi
H
tp hp các
đim biu din hình hc ca s phc
z
tha mãn
12
4 3 2 2
zz
zi

. Din tích ca hình phng
H
A.
44
. B.
88
. C.
24
. D.
84
.
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u 148.(THPT Chuyên Hà Tĩnh 2019) Cho các s phc
1
z
,
2
z
thỏa mãn phương trình
12
6zz
2 3 5zi
. Biết rng tp hợp các đim biu din s phc
12
w z z
một đường tròn.
Tính bán kính đường tròn đó.
A.
8R
. B.
4R
. C.
22R
. D.
2R
.
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
Li gii
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u 149.(Chuyên Đại Hc Vinh 2019) Cho các s phc tha mãn .
Tp hợp các điểm biu din s phc trên mt phng tọa độ một đưng thng.
Khong cách t đến đường thẳng đó bằng
A. . B. . C. . D. .
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u 150.(THPT Chuyên Thái Nguyên 2019) Cho hai số phức
12
,zz
khác
0
, thỏa mãn
22
1 2 1 2
z z z z
.
,MN
lần lượt hai điểm biểu diễn số phức
12
,zz
trên mặt phẳng
Oxy
. Mệnh đề nào sau đây
đúng?
A. Tam giác
OMN
nhọn và không đều. B. Tam giác
OMN
đều.
C. Tam giác
OMN
tù. D. Tam giác
OMN
vuông.
Li gii
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z
2020
2 1 2z i z i
2 1 4w z i
2; 3I
18 5
5
18 13
13
10 3
3
10 5
5
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 151.(THPT Thanh Chương 2019) Cho s phc
z
tha mãn
44z z z z
và s phc
2 2 4w z i zi i
phn o s thực không dương. Trong mt phng tọa độ
Oxy
, hình
phng
H
tp hợp các đim biu din ca s phc
z
. Din tích hình
H
gn nht vi s nào
sau đây?
A.
7
. B.
17
.
C.
21
. D.
193
.
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u 152.(THPT Đoàn Thượng 2019) Trong mt phng tọa độ
Oxy
gi hình
()H
tp hp các
đim biu din s phc
z
tha mãn
| 2 | 2
10
zi
xy
. Tính din tích
()S
ca hình phng
()H
A.
4S
. B.
1
4
S
. C.
1
2
S
. D.
2S
.
Li gii
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u 153.(THPT Nguyn Công Tr 2019) Cho
12
,z z
là hai s phc tha mãn
22z i iz
, biết
12
1zz
. Tính giá tr ca biu thc
12
P z z
.
A.
3
2
. B.
3
. C.
2
. D.
2
2
.
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 154.(SGD&ĐT Kiên Giang 2019) Cho hai số phức
12
,zz
thỏa mãn điều kiện
12
4, 6zz
12
10zz
. Giá trị của
12
2
zz
A.
1
. B.
0
. C.
2
. D.
3
.
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u 155.(Tp Chí Tn Hc Tui Tr 2019) Có bao nhiêu s phc
z
tha mãn
2
32z z z
4 3 3zi
?
A.
1
. B.
2
. C.
3
. D.
4
.
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 156.(THPT Chuyên Quang Trung 2019) Gi
S
là tp tt c các giá tr thc ca tham s
m
để
tn ti 4 s phc
tha mãn
2z z z z
2z z z z m
s thun o. Tng các
phn t ca
S
là.
A.
21
. B.
21
2
. C.
3
2
. D.
1
2
.
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u 157.(SGD & ĐT Nam Định 2018).Cho hai số phức
12
,zz
thoả mãn
1
6z
,
2
2z
. Gọi
M
,
N
là các điểm biểu diễn cho
1
z
2
iz
. Biết
60MON 
. Tính
22
12
9T z z
.
A.
18T
. B.
24 3T
. C.
36 2T
. D.
36 3T
.
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u 158.(Cụm 5 Chuyên Đồng Bằng Sông Cửu Long 2018) . Cho hai số phức
12
,zz
thoả mãn
12
2, 3zz
. Gọi
M
,
N
các điểm biểu diễn cho
1
z
và
2
iz
. Biết
30MON 
. Tính
22
12
4S z z
.
A.
52
. B.
33
. C.
47
. D.
5
.
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u 159. Xét các số phức
; z x yi x y
có tập hợp điểm biểu diễn trên mặt phẳng tọa độ là
đường tròn phương trình
22
: 1 2 4C x y
. Tìm tập hợp các điểm biểu diễn của số
phức
2w z z i
.
A. Đường thẳng. B. Đoạn thẳng. C. Điểm. D. Đường tròn.
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u 160. Gọi
1
z
2
z
các nghim của phương trình
2
4 9 0zz
. Gọi
,,M N P
lần lượt các
điểm biểu diễn của
1
z
,
2
z
số phức
; w x yi x y
trên mặt phẳng tọa độ. Khi đó tập hợp
điểm
P
trên mặt phẳng phức để tam giác
MNP
vuông tại
P
là:
A. Đường thẳng có phương trình
22
2 1 0x x y
B. Là đường tròn có phương trình
2
2
2 5.xy
C. Là đường tròn có phương trình
2
2
25xy
nhưng không chứa
,MN
.
D. Là đường tròn có phương trình
22
2 1 0x x y
nhưng không chứa
,MN
.
Li gii
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u 161. Trong mặt phẳng tọa độ, cho số phức
z
thỏa mãn điều kiện
3 4 2zi
. Tập hợp các
điểm biểu diễn số phức
21w z i
là hình tròn có diện tích
S
bằng:
A.
19 .S
B.
12 .S
C.
16 .S
D.
25 .S
Li gii
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u 162. Cho
,zw
các số phức thỏa mãn
1, 1z z w
. Tìm tập hợp các điểm biểu diễn của
số phức
w
.
A. Hình tròn
22
: 4.C x y
B. Đường tròn
22
: 4.C x y
C. Hình tròn
2
2
: 1 4.C x y
D. Đường tròn
2
2
: 1 4.C x y
Li gii
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u 163. Có bao nhiêu số phức
thỏa mãn
3 6 5zi
1 2 1 12 15i z i
?
A.
. B.
1.
C.
2.
D. Vô số.
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u 164.(Đề Chính Thc BGD 2017) Gi
S
tp hp tt c các giá tr thc ca tham s
m
để tn
ti duy nht s phc
thỏa mãn điều kin
.1zz
3z i m
. Tìm s phn t ca
S
.
A.
2.
B.
4.
C.
1.
D.
3.
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u 165. (THPT Chuyên Quang Trung 2018) Cho số phức
thỏa mãn
2 3 2 3z i z i
. Biết
1 2 7 4 6 2z i z i
,
;M x y
là điểm biểu diễn số phức
z
, khi đó
thuộc khoảng
A.
0;2
. B.
1;3
. C.
4;8
. D.
2;4
.
Li gii
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II. CỰC TRCỦA SỐ PHỨC.
1. Nhận xét: Trong nhóm bài toán tìm giá trị lớn nhất giá trị nhỏ nhất của biểu thức số phức
nhiều phương pháp giải, nhưng không có công cụ nào gọi “vạn năng” để giải quyết hết tất cả
các bài toán. Tùy vào đặc điểm của từng đề bài mà ta chọn phương pháp phù hợp sao cho nhanh,
gọn, phù hợp với trắc nghiệm. Nhưng trước tiên ta cần nắm vững thật kỹ các phương pháp.
Ta có thể sử dụng phương pháp hàm số (hoặc tam thức) để tìm max – min.
Phương pháp hình học .
Phương pháp lượng giác hóa.
Phương pháp bất đẳng thức.
2. Bài toán: Cho các số phức
( , )z x yi x y
thỏa mãn điều kiện
1
. Tìm giá trị lớn nhất và nhỏ
nhất của
2 .fz
3. Mt s bài toán tìm cc tr phương pháp.
Bài toán 1. Nếu tập hợp là một đường một đường thẳng
0Ax By C
1. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
. , .z x y i x y
c 2. Biến đổi điều kin
1
ban đầu để tìm đường đưng thng
.0Ax By C
c 3. Ta s dng hai cách sau
ch 1. Phương pháp hàm số:
T
0
C Ax
Ax By C y
B
ri thay vào s phc
2fz
theo biến
x
và kho sát
hàm s tìm được giá tr ln nht và giá tr nh nht.
ch 2. Phương pháp hình học:
Cho đường thng
( ): 0Ax By C
và điểm
( ; ).M x y
Đim
()H 
sao cho
MH
nh nht thì
H
là hình chiếu
vuông góc ca
M
lên
( ).
,( )
min
22
O
C
z OH d
AB
Khi đó
MH
và tọa độ
( ) ( ).H OH
[ ;( )]
min
22
( )
N
Ax By C
z x y i NK d
AB

`
Khi đó
MK
và tọa độ
( ) ( ).K MK
Đặc bit: Nếu điểm biu din thuộc đoạn thng thì ta xét 2
trường hp
Trường hp 1. Nếu tam giác
ABI
IAB
tù hoc
ABI
thì
in
;
m
MI Min IA IB
.
;
Max
MI Max IA IB
.
Trường hp 2. Nếu tam giác
ABI
IAB
tù và
ABI
đều
không tù thì
in
;.
m
MI d I AB
;
Max
MI Max IA IB
.
B
I
A
M
B
I
A
M
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2. i tập minh họa.
Bài tp 23.Trong các số phức thỏa mãn
2 4 2 ,z i z i
tìm số phức có môđun nhỏ nhất ?
Li gii.
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Bài tp 24. Xét các số phức
, ( , )z x yi x y
thỏa mãn
2 4 2z i z i
z
đạt giá tr
nhỏ nhất. Tìm
3 2 .P x y
Li gii.
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Bài tp 25. Cho s phc
tha
3 1 3u z i z i
là mt s thc. Giá tr nh nht ca
z
Li gii.
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Bài tp 26. Cho s phc
tha mãn:
2 3 1 2 z i z i
. Tìm giá tr nh nht ca
3zi
.
Li gii.
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Bài tp 27. Cho s phc
z
tha mãn:
2
4 ( 2 ) z z z i
. Tìm giá tr nh nht ca
zi
Li gii.
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Bài tp 28. Cho các số phức
thỏa mãn
2 2 4 .z i z i
Tìm giá trị nhỏ nhất của
1.iz
Li gii.
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3. u hỏi trắc nghiệm.
Mc độ 2. Thông hiu
u 166.(THPT Chuyên Bến Tre 2017) Trong các số phức thỏa mãn điều kiện:
2 4 2z i z i
.
Tìm số phức
có môđun nhỏ nhất.
A.
2zi
. B.
3zi
. C.
22zi
. D.
13zi
.
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u 167.(THPT Nguyễn Du 2019) Số phức
z
có môđun nhỏ nhất thỏa mãn
23i z z i
A.
36
55
i
. B.
36
55
i
. C.
63
55
i
. D.
63
55
i
.
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u 168.(S GD & ĐT Nam Định 2019) Trong các s phc
z
tha mãn
12 5 17 7
13
2
i z i
zi

.
Tìm giá tr nh nht ca
z
.
A.
3 13
26
. B.
5
5
. C.
1
2
. C.
2
.
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u 169.(THPT Bình Xuyên 2018) Trong các số phức
thỏa mãn
2 4 2z i z i
. Số phức
có môđun nhỏ nhất là
A.
1zi
. B.
22zi
. C.
22zi
. D.
32zi
.
Li gii
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u 170.(THPT Chuyên Nguyên Giáp 2017) Biết số phức
,,z a bi a b
thỏa mãn điều
kiện
2 4 2z i z i
có mô đun nhỏ nhất. Tính
22
M a b
.
A.
16M
. B.
26M
. C.
10M
. D.
8M
.
Li gii
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u 171.(THPT Hàm Long 2017) Trong các số phức
thỏa điều kiện
24z z i
, số phức
môđun nhỏ nhất là.
A.
5z
. B.
5
2
zi
. C.
12zi
. D.
3zi
.
Li gii
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u 172.(THPT Thường Kiệt 2018) Trong mặt phẳng với hệ toạ độ
,Oxy
cho điểm
4; 4A
M
là điểm biển diễn số phức
z
thoả mãn điều kiện
12z z i
. Tìm toạ độ điểm
M
để đoạn
thẳng
AM
nhỏ nhất.
A.
1; 1M 
. B.
2; 4M 
. C.
1; 5M
. D.
2; 8M
.
Li gii
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u 173.(THPT Lương Tài 2018) Trong các số phức thỏa mãn điều kiện
2 3 1 2z i z i
, hãy
tìm phần ảo của số phức có môđun nhỏ nhất ?
A.
10
13
. B.
2
5
. C.
2
. D.
2
13
.
Li gii
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Mc độ 3. Vn dng
u 174.(THPT Nguyễn Huệ-Huế) Cho số phức
thỏa mãn
13z i z i
.
Tính môđun nhỏ nhất của
zi
.
A.
35
10
. B.
45
5
. C.
35
5
. D.
75
10
.
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u 175.(THPT chuyên Lương Thế Vinh 2018)
Cho số phức
thỏa mãn
2
2 5 1 2 3 1z z z i z i
. Tính
min | |w
, với
22w z i
.
A.
3
min | |
2
w
. B.
1
min | |
2
w
. C.
min | | 1w
. D.
min | | 2w
.
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 176.(Cụm 1 Hồ Chí Minh 2018) Cho số phức
z
thỏa điều kiện
2
42z z z i
. Giá trị nhỏ
nhất của
zi
bằng ?
A. 3. B. 4. C. 1. D. 2.
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u 177.(Cụm THPT ng u 2019) Cho số phức
thỏa mãn hệ thức
25z i z i
1zi
nhỏ nhất. Tổng phần thực và phần ảo của
z
bằng
A.
16
5
. B.
3
5
. C.
11
5
. D.
11
5
.
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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Đặc bit: Cho đường thng
d
c định hai đim
,AB
c định
không nm trên
d
. Một điểm
M
thay đổi trên
d
. Khi đó:
Trường hp 1. Nếu
,AB
thuc hai na mt phng khác nhau b
là đường thng
d
thì
min
MA MB AB
khi
.M AB d
Trường hp 2. Nếu
,AB
thuc cùng mt na mt phng b
đưng thng
d
thì
min min
MA MB MA MB A B

khi
M A B d

vi
A
là điểm đối xng vi
A
qua đường thng
d
u 178. Nếu
z
là số phức thỏa mãn
2z z i
thì giá trị nhỏ nhất của
4z i z
A.
4
. B.
2
. C.
3
. D.
5
.
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u 179.(THPT Năng Khiếu TP HChí Minh 2018) Nếu
z
số phức thỏa
2z z i
thì giá trị
nhỏ nhất của
4z i z
A.
. B.
3
. C.
. D.
5
.
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d
B
A
M
M'
d
H
A
B
A'
M
M'
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 180.(ĐHQG TPHCM-2018) Nếu
số phức thỏa
2z z i
thì giá trị nhỏ nhất của
4z i z
A.
. B.
3
. C.
. D.
5
.
Li gii
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u 181.(Sở GD & ĐT Trà Vinh 2018) Xét các số phức
z a bi
,
,ab
thỏa mãn đồng thời
hai điều kiện
43z z i
1 2 3z i z i
đạt giá trị nhỏ nhất. Giá trị
2P a b
là:
A.
252
50
P 
B.
41
5
P 
. C.
61
10
P 
. D.
18
5
P 
.
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u 182.(Tp Chí Toán Hc 2019) Cho số phức
z
thỏa mãn
4 3 4 4 5z i z i
. Tìm giá trị nhỏ
nhất của biểu thức
3P z i z i
.
A. min
22P
. B. min
25P
. C. min
52P
. D. min
5P
.
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u 183.(Sở GD & ĐT Nam 2019) Cho số phức
z a bi
với
,ab
là hai số thực thỏa mãn điều
kiện
21ab
. Tính
z
khi biểu thức
1 4 2 5z i z i
đạt giá trị nhỏ nhất.
A.
1
5
. B.
5
. C.
1
5
. D.
2
5
.
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Mc độ 4. Vn dng Cao.
u 184.(THPT Kim Liên 2019) Xét các s phc
z
tha mãn
3 2 3 3 5z i z i
. Gi
,Mm
lần lượt là hai giá tr ln nht và nh nht ca biu thc
2 1 3P z z i
. Tìm
,Mm
.
A.
17 5, 3 2Mm
. B.
26 2 5, 2Mm
.
C.
26 2 5, 3 2Mm
. D.
17 5, 2Mm
.
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u 185. Chính Thc BGD 2017) Xét các s phc
tha mãn
2 4 7 6 2.z i z i
Gi
, mM
lần lượt là giá tr nh nht và giá tr ln nht ca
1zi
. Tính
.P m M
A.
13 73P 
. B.
5 2 2 73
2
P
. C.
5 2 2 73P 
. D.
5 2 73
2
P
.
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u 186. Cho s phc
z
tha mãn
1 3 2 5 z i z i
. Gi
,Mm
lần lượt giá tr ln nht
và giá tr nh nht ca
z
. Tính
Mm
.
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u 187. Xét s phc
tha mãn
3 2 3 3 5.z i z i
Gi
, Mm
ln lượt giá tr ln nht
và giá tr nh nht ca biu thc
2 1 3P z z i
.
A.
17 5, 3 2.Mm
B.
26 2 5, 3 2.Mm
C.
26 2 5, 2.Mm
D.
17 5, 2.Mm
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u 188. Xét s phc
tha mãn
2 3 6 2 17.z i z i
Gi
, Mm
lần lượt giá tr ln
nht và giá tr nh nht ca biu thc
1 2 2P z i z i
.
A.
3 2, 0.Mm
B.
3 2, 2.Mm
C.
3 2, 5 2 2 5.Mm
D.
2, 5 2 2 5.Mm
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u 189. Xét s phc
z
tha mãn
2 2 1 3 34.z i z i
Tìm giá tr nh nht ca bin thc
1.P z i
A.
min
9
.
34
P
B.
min
3.P
C.
min
13.P
D.
min
4.P
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Bài toán 7. Nếu tập hợp là một đường một đường tròn
2 2 2
22
( ) ( )
2 2 0
x a y b R
x y ax by c
1. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
. , .z x y i x y
c 2. Biến đổi điều kiện ban đầu để tìm đường tròn
2 2 2
22
( ) ( )
2 2 0
x a y b R
x y ax by c
c 3. Ta s dng hai cách sau
ch 1. Phương pháp hình học:
Cho tp hợp các điểm
( ; )M x y
biu din s phc
z x yi
( , )xy
là mt đưng tròn
()C
có tâm
( ; )I a b
và bán
kính
.R
Gi
N
là điểm biu din s phc
.z
Khi đó:
min 1 1
min
max 2 2
max
khi
khi
z OM OM OI R M M
z OM OM OI R M M
Khi đó
12
( ) ( ) { ; }.OI C M M
min 1 1
min
max 2 2
max
khi
khi
z z MN NN NI R M N
z z MN NN NI R M N
Khi đó
12
( ) ( ) { ; }.NI C N N
ch 2. Phương pháp ng giác hóa:
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Đối vi nhóm bài toán mà tp hợp điểm biu din s phc là một đường tròn thì việc lượng
giác hóa t ra khá hiu qu và nhanh chóng.
Gi s có được gi thiết
22
2 2 2
( ) ( ) 1,
x a y b
x a y b R
RR

s gợi ta đến công
thc
22
sin cos 1tt
nên đặt
sin
sin
cos
cos
xa
t
x R t a
R
y b y R t b
t
R


để đưa bài toán về dạng lượng
giác quen thuc. Ngoài ra, ta cn nh những đánh giá thường được s dng:
1 sin 1,t
1 cos 1t
22
sin cos sin( ).a t b t a b t
Bất đẳng thc Cauchy Schwarz dng 1:
2 2 2 2
( )( ).ax by a b x y
2 2 2 2 2 2
sin cos ( )(sin cos ) .a t b t a b t t a b
Du
""
xy ra khi và ch khi
22
sin cos
sin cos
tt
ab
a t b t a b
2 2 2 2 2 2
sin cos ( )(sin cos ) .a t b t a b t t a b
Du
""
xy ra khi và ch khi
22
sin cos
sin cos
tt
ab
a t b t a b
2. i tập minh họa.
Bài tp 29. Cho số phức
thỏa
3 4 4.zi
Tìm giá trị lớn nhất
max
P
của
.Pz
Li gii.
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Nhn xét: Cho đường tròn
C
tâm
O
, bán kính
R
điểm
c định. Một điểm
M
thay đổi
trên
C
. Khi đó
Nếu
I
nm ngoài
C
thì
inm
MI OI R
,
max
MI OI R
Nếu
I
nm trong
C
thì
inm
MI R OI
,
max
MI OI R
.
Nếu
nm trên
C
thì
in max
0, 2
m
MI MI R
.
Bài tp 30. Cho các số phức
( , )z x yi x y
thỏa mãn
(2 4 ) 2.zi
Gọi
12
, zz
lần lượt là
hai số phức có môđun lớn nhất và môđun nhỏ nhất. Tính tổng phần ảo của hai số phức
12
, zz
đó.
Li gii
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Bài tp 31. Cho số phức
( , )z x yi x y
thỏa mãn đồng thời các điều kiện
2 3 1zi
biểu thức
1zi
đạt giá trị lớn nhất. Tính giá trị của biểu thức
3 2 .xy
Li gii
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Bài tp 32. Cho s phc z tha mãn
2
2
1


zi
zi
. Tìm giá tr nh nht và ln nht ca
z
.
Li gii
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Bài tp 33. Cho s phc
tha mãn
1
3
3 4 1
log 1
2 3 4 8




zi
zi
.
Tìm giá tr nh nht và ln nht ca
z
.
Li gii
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Bài tp 34. Cho s phc z tha mãn:
2
6 25 2 3 4 z z z i
. Tìm giá tr nh nht giá tr
ln nht ca
35zi
Li gii
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Bài tp 35. Cho s phc
tha mãn
3 3 2
1 2 3
1 2 2
i
zi
i
. Tìm giá tr nh nht ln nht
ca
32zi
Li gii
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3. u hỏi trắc nghiệm.
Mc độ 3. Vn dng
u 190.(THPT Hùng Vương 2017) Cho số phức
thỏa mãn điều kiện
11z i z
.
Đặt
mz
, tìm giá trị lớn nhất của
m
.
A. 1. B.
2
. C.
21
. D.
21
.
Li gii
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u 191.(THPT chuyên ĐHKH Huế) Trong các số phức
thỏa
3 4 2zi
, gọi
0
z
số phức
mô đun nhỏ nhất. Khi đó.
A.
0
7z
. B.
0
2z
.
C.
0
3z
. D. Không tồn tại số phức
0
z
.
Li gii
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Câu 192.(THPT chuyên Lam Sơn 2019) Cho số phức
, tìm giá trị lớn nhất của
z
biết rằng
tha
mãn điều kiện
23
11
32
i
z
i


.
A.
1
. B.
. C.
3
. D.
2
.
Li gii
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u 193.(THPT Tiên Du 2017) Cho s phc
tha mãn
2 4 5zi
và
min
z
.
Khi đó s phc
A.
32zi
. B.
2zi
. C.
12zi
. D.
4 5 .zi
Li gii
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u 194.(THPT Thái Phiên 2019) Trong tập hợp các số phức
thỏa mãn:
2
2.
1
zi
zi


Tìm môđun lớn nhất của số phức
.zi
A.
22
. B.
32
. C.
32
. D.
22
.
Li gii
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u 195.(THPT TH Cao Nguyên 2019) Cho số phức thỏa mãn
2 2 1zi
.
Giá trị lớn nhất của
z
A.
4 2 2
. B.
22
. C.
2 2 1
. D.
3 2 1
.
Li gii
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u 196.(THPT Thanh Thủy 2018) Trong mặt phẳng tọa độ, hãy tìm số phức
z
môđun nhỏ
nhất, biết rẳng số phức
z
thỏa mãn điều kiện
2 4 5zi
.
A.
12zi
. B.
12zi
. C.
12zi
. D.
12zi
.
Li gii
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u 197.(S GD & ĐT Long An 2017) Cho s phc
tha mãn
2 3 1zi
.
Tìm giá tr ln nht ca
z
.
A.
2 13
. B.
13 1
. C.
13
. D.
1 13
.
Li gii
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u 198.(THPT Gia Lộc 2017) Cho số phức
z
thỏa mãn
3 4 1zi
. Tìm giá trị nhỏ nhất của
z
A.
.
B.
3
. C.
5
. D.
.
Li gii
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u 199.(THPT Kim Liên 2019) Cho số phức
thỏa mãn
2 3 1zi
.
Tìm giá trị lớn nhất của
1zi
.
A.
. B.
13 1
. C.
13 2
. D.
.
Li gii
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u 200.(THPT chuyên Phan Bội Châu) Cho số phức
thỏa mãn
2 3 1 zi
.
Giá trị lớn nhất của
1zi
là.
A.
. B.
13 1
. C.
13 2
. D.
.
Li gii
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u 201.(THPT chuyên Nguyễn Trãi 2017) Cho số phức
z
thỏa mãn:
2 2 1zi
. Số phức
zi
có môđun nhỏ nhất là:
A.
51
. B.
51
. C.
52
. D.
52
.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
u 202.(THPT Chuyên Sơn La 2019) Cho số phức
thỏa mãn điều kiện :
1 2 5zi
và
1w z i
có môđun lớn nhất. Số phức
có môđun bằng:
A.
6
. B.
32
. C.
52
. D.
25
.
Li gii
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u 203.(Cụm 6 Hồ Chí Minh) Cho số phức
z
thỏa mãn
32zz
max 1 2 2z i a b
.
Tính
ab
.
A.
42
. B.
3
. C.
4
3
. D.
4
.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 204.(THPT Chuyên H Long 2019) Cho số phức
z a bi
, ab
thỏa mãn:
2
2
z
zi
một
số thuần ảo. Khi số phức
z
có môđun lớn nhất, hãy tính
ab
.
A.
2 2 1ab
. B.
4ab
. C.
4ab
. D.
22ab
.
Li gii
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u 205.(THPT Đoàn Thượng 2019) Cho s phc
tha mãn
2.z
Giá tr nhỏ nht ca biu
thc
34P z i
bng:
A. 5. B. 3. C. -3. D. 7.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
u 206.(Sở GD & ĐT Vĩnh Phúc 2019) Cho số phức
thỏa mãn
2 3 1zi
.Giá trị lớn nhất của
1zi
A. 4 B. 6 C.
13 1
. D.
13 2
.
Li gii
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u 207.(THPT Nguyễn Khuyến2019) Xét số phức
thỏa mãn
2 1 3 2 2.z z i
Mệnh đề
nào dưới đây đúng?
A.
2z
.
B.
1
2
z
. C.
13
22
z
. D.
3
2
2
z
.
Li gii
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Bài toán 8. Nếu tập hợp là một đường một đường Elíp
:E
22
22
1.
xy
ab

1. Nhận dạng trắc nghiệm.
Khi gp gi thiết s phc có dng
1 1 2 2
2z a bi z a b i a
1.
Ta nghĩ ngay tập hợp biểu diễn của số phức
điểm
;M x y
nằm trên Elíp nếu
2a AB
với
1 1 2 2
; , ; .A a b B a b
Khi đó
E
nhận
1 1 2 2
; , ;A a b B a b
làm hai tiêu điểm và độ dài trục lớn là
2.a
2. Phương pháp.
c 1. Gi
;M x y
là điểm biu din ca s phc
.,z x y i x y
c 2. Biến đổi điều kin
1
để tìm mi liên ca
x
y
giống như các dng trên.
c 3. Kết lun:
Giá trị lớn nhất của môđun
a
.
Giá trị nhỏ nhất của môđun
.b
3. Kiến thức bỗ tr.
Định nghĩa: Cho hai điểm cố định
1
F
2
F
với
12
2 0.F F c
Đường elip tập hợp các điểm
M
sao cho
12
2 , ( ).MF MF a a c
Hai điểm
12
, FF
gọi là các tiêu điểm của elip.
Khoảng cách
2c
được gọi là tiêu cự của elip.
Phương trình chính tắc của elip:
22
22
( ): 1
xy
E
ab

với
0.ab
c thông số cần nhớ:
Trục lớn
12
2.A A a
Trục bé
12
2.B B b
Tiêu cự
12
2.F F c
Mối liên hệ
2 2 2
.a b c
Bán kính qua tiêu của
M
1 2 1 2
, 2 .
cc
MF a x MF a x MF MF a
aa
4. i tập minh họa.
Bài tp 36. (THPT Thuận Thành 2019) Cho số phức
thỏa mãn
4 4 10.zz
Giá trị lớn nhất và nhỏ nhất của
z
lần lượt là.
A.
10 và 4
. B.
5 và 4
. C.
4 và 3
. D.
5 và 3
.
Li gii
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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Bài tp 37.(THPT Chuyên Tĩnh 2019) Cho số phức
thỏa mãn
3 3 10z i z i
.
Gọi
1
M
,
2
M
lần lượt điểm biểu diễn số phức
môđun lớn nhất nhỏ nhất. Gọi
M
trung điểm của
12
MM
,
;M a b
biểu diễn số phức
w
, tổng
ab
nhận giá trị nào sau đây?
A.
7
2
. B.
5
. C.
. D.
9
2
.
Li gii
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u 38.(Sở GD & ĐT Bắc Ninh 2018) Cho số phức
z
thỏa mãn
2 2 5zz
.
Gi
,Mm
lần lượt là giá tr ln nht, giá tr nh nht ca
z
. Tính
Mm
?
A.
17
2
Mm
. B.
8Mm
. C.
1Mm
. D.
4Mm
.
Li gii
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Trung Tâm Luyện Thi Đại Học Amsterdam Chương IVBài 2. Tập Hợp Điểm-Cực Tr
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Lớp Toán Thầy-Diệp Tn Tel: 0935.660.880
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u 39.(THPT Chuyên Nguyn Quang Diêu 2019)
Gi
S
tp hp các s phc tha
3 3 10zz
. Gi
12
;zz
hai s phc thuc
S
đun
nh nht. Giá tr biu thc
22
12
P z z
A.
16
. B.
16
. C.
32
. D.
32
.
Li gii
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u 40.(THPT Chuyên H Long 2019) Cho số phức
z
thỏa mãn
6 6 20zz
. Gọi
M
,
n
lần
lượt là môđun lớn nhất và nhỏ nhất của z. Tính
Mn
A.
2Mn
. B.
4Mn
. C.
7Mn
. D.
14Mn
.
Li gii
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