4-1.Complex numbers
hard

If the least and the largest real values of $\alpha,$ for which the equation $z+\alpha|z-1|+2 i=0$ $( z \in C$ and $i=\sqrt{-1}$ ) has a solution, are $p$ and $q$ respectively; then $4\left( p ^{2}+ q ^{2}\right)$ is equal to ..........

A

$15$

B

$10$

C

$20$

D

$5$

(JEE MAIN-2021)

Solution

Put $z=x+$ iy

$x+i y+\alpha \mid x+$ iy $-1 \mid+2 i=0$

$\Rightarrow \quad x+\alpha \sqrt{(x-1)^{2}+y^{2}}+i(y+2)=0+0 i$

$\Rightarrow \quad y+2=0$ and $x+\alpha \sqrt{(x-1)^{2}+y^{2}}=0$

$\Rightarrow \quad y=-2$ and $\alpha^{2}=\frac{x^{2}}{x^{2}-2 x+5}$

Now $\frac{x^{2}}{x^{2}-2 x+5} \in\left[0, \frac{5}{4}\right]$

$\therefore \quad \alpha^{2} \in\left[0, \frac{5}{4}\right] \Rightarrow \alpha \in\left[-\frac{\sqrt{5}}{2}, \frac{\sqrt{5}}{2}\right]$

$\Rightarrow \quad 4\left(p^{2}+q^{2}\right)=4\left(\frac{5}{4}+\frac{5}{4}\right)=10$

Standard 11
Mathematics

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