Let $$S=\left\{z \in \mathbb{C}: \bar{z}=i\left(z^{2}+\operatorname{Re}(\bar{z})\right)\right\}$$. Then $\sum_\limits{z \in \mathrm{S}}|z|^{2}$ is equal to :
Solution
Let $z=x+i y$
<br/><br/>$$
\begin{aligned}
&\bar{z}=i\left(z^2+\operatorname{Re}(\bar{z})\right) \\\\
&\Rightarrow x-i y=i\left(x^2-y^2+2 i x y+x\right) \\\\
& x-i y=i\left(x^2-y^2+x\right)-2 x y \\\\
&x=-2 x y \Rightarrow x(2 y+1)=0 \\\\
&\Rightarrow x=0, y=\frac{-1}{2} ........(i)\\\\
&-y=x^2-y^2+x ...........(ii)
\end{aligned}
$$
<br/><br/><b>Case (I)</b> $x=0$
<br/><br/>$\Rightarrow-y=-y^2 \Rightarrow y^2-y=0 \Rightarrow y=0,1$
<br/><br/>$\therefore$ $z=0, i$
<br/><br/><b>Case (II)</b> $y=\frac{-1}{2}$
<br/><br/>$\Rightarrow \frac{1}{2}=x^2-\frac{1}{4}+x \Rightarrow x^2+x-\frac{3}{4}=0$
<br/><br/>$\Rightarrow$ $4 x+4 x-3=0 \Rightarrow(2 x-1)(2 x+3)=0$
<br/><br/>$\Rightarrow$ $x=\frac{1}{2}, \frac{-3}{2}$
<br/><br/>$$
\begin{aligned}
& z=\frac{1}{2}-\frac{1}{2} i, \frac{-3}{2}-\frac{1}{2} i \\\\
& \sum|z|^2=0+1+\frac{1}{2}+\frac{5}{2}=4
\end{aligned}
$$
About this question
Subject: Mathematics · Chapter: Complex Numbers and Quadratic Equations · Topic: Complex Numbers and Argand Plane
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