Medium MCQ +4 / -1 PYQ · JEE Mains 2023

For two non-zero complex numbers $z_{1}$ and $z_{2}$, if $\operatorname{Re}\left(z_{1} z_{2}\right)=0$ and $\operatorname{Re}\left(z_{1}+z_{2}\right)=0$, then which of the following are possible?

A. $\operatorname{Im}\left(z_{1}\right)>0$ and $\operatorname{Im}\left(z_{2}\right) > 0$

B. $\operatorname{Im}\left(z_{1}\right) < 0$ and $\operatorname{Im}\left(z_{2}\right) > 0$

C. $\operatorname{Im}\left(z_{1}\right) > 0$ and $\operatorname{Im}\left(z_{2}\right) < 0$

D. $\operatorname{Im}\left(z_{1}\right) < 0$ and $\operatorname{Im}\left(z_{2}\right) < 0$

Choose the correct answer from the options given below :

  1. A A and C
  2. B A and B
  3. C B and D
  4. D B and C Correct answer

Solution

<p>Let, ${z_1} = {x_1} + i{y_1}$</p> <p>and ${z_2} = {x_2} + i{y_2}$</p> <p>$\therefore$ ${z_1}{z_2} = {x_1}{x_2} - {y_1}{y_2} + i({x_1}{y_2} + {x_2}{y_1})$</p> <p>Given, ${\mathop{\rm Re}\nolimits} ({z_1} + {z_2}) = 0$</p> <p>$\Rightarrow {x_1} + {x_2} = 0$ ...... (1)</p> <p>also given, ${\mathop{\rm Re}\nolimits} ({z_1}{z_2}) = 0$</p> <p>$\Rightarrow {x_1}{x_2} - {y_1}{y_2} = 0$</p> <p>$\Rightarrow {x_1}{x_2} = {y_1}{y_2}$</p> <p>$\Rightarrow {y_1}{y_2} = - x_1^2$ [$\because$ ${x_2} = - {x_1}$]</p> <p>So, multiplication of imaginary part's of z<sub>1</sub> and z<sub>2</sub> is negative. It means sign of y<sub>1</sub> and y<sub>2</sub> are opposite of each other.</p> <p>$\therefore$ B and C are correct.</p>

About this question

Subject: Mathematics · Chapter: Complex Numbers and Quadratic Equations · Topic: Complex Numbers and Argand Plane

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