Trong các số phức \[{z_1} = - 2i,\,\,{z_2} = 2 - i,\,\,{z_3} = 5i,\,\,{z_4} = 4\] có bao nhiêu số thuần ảo?

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.\(\sqrt 2 \)