Chọn mệnh đề đúng:

A. 13

B. 25

C. 7

D. 19

A.$\frac{1}{z} = \frac{1}{2} - \frac{{\sqrt 3 }}{2}i$

B. $\frac{1}{z} = \frac{1}{2} + \frac{{\sqrt 3 }}{2}i$

C. $\frac{1}{z} = \frac{1}{4} + \frac{{\sqrt 3 }}{4}i$

D. $\frac{1}{z} = \frac{1}{4} - \frac{{\sqrt 3 }}{4}i$

A.a−bi          

B.a+bi 

C.b−ai          

D.b+ai

A.$\left\{ {\begin{array}{*{20}{c}}{x = - \frac{1}{7}}\\{y = - \frac{4}{7}}\end{array}} \right.$

B. $\left\{ {\begin{array}{*{20}{c}}{x = \frac{4}{7}}\\{y = \frac{1}{7}}\end{array}} \right.$

C. $\left\{ {\begin{array}{*{20}{c}}{x = - \frac{4}{7}}\\{y = \frac{1}{7}}\end{array}} \right.$

D. $\left\{ {\begin{array}{*{20}{c}}{x = 0}\\{y = 0}\end{array}} \right.$

A.z=i                             

B.z=1+i                          

C.z=1−i                     

D.z=1

A.a

B.b

C.i

D.z

A.−1

B.2

C.1

D.$\sqrt 2$