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 :
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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