Medium MCQ +4 / -1 PYQ · JEE Mains 2020

Let z be complex number such that
$\left| {{{z - i} \over {z + 2i}}} \right| = 1$ and |z| = ${5 \over 2}$.
Then the value of |z + 3i| is :

  1. A $2\sqrt 3$
  2. B $\sqrt {10}$
  3. C ${{15} \over 4}$
  4. D ${7 \over 2}$ Correct answer

Solution

Given $\left| {{{z - i} \over {z + 2i}}} \right| = 1$ <br><br>|z – i| = |z + 2i| <br><br>(let z = x + iy) <br><br>$\Rightarrow$ x<sup>2</sup> + (y – 1)<sup>2</sup> = x<sup>2</sup> + (y + 2)<sup>2</sup> <br><br>$\Rightarrow$ y = $- {1 \over 2}$ <br><br>Also given |z| = ${5 \over 2}$ <br><br>$\Rightarrow$ x<sup>2</sup> + y<sup>2</sup> = ${{25} \over 4}$ <br><br>$\Rightarrow$ x<sup>2</sup> = 6 <br><br>$\therefore$ z = $\pm \sqrt 6$ - $- {1 \over 2}i$ <br><br>|z + 3i| = $\sqrt {6 + {{25} \over 4}}$ = ${7 \over 2}$

About this question

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

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