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Question 25 — JEE (Main) 4 April 2024 (S I)

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  • #11808
    Om Sharma
    Keymaster

    Let $\alpha$ and $\beta$ be the sum and the product of all the non-zero solutions of the equation $(\bar{z})^{2}+|z|=0, z \in C$. Then $4\left(\alpha^{2}+\beta^{2}\right)$ is equal to
    [JEE (Main) 4 April 2024 S I]
    (A) 6
    (B) 8
    (C) 4
    (D) 2

    #11809
    Om Sharma
    Keymaster

    Solution

    let $\mathrm{z}=\mathrm{x}+\mathrm{iy}, \overline{\mathrm{z}}=\mathrm{x}-\mathrm{iy} \Rightarrow \overline{\mathrm{z}}^{2}=\mathrm{x}^{2}-\mathrm{y}^{2}-2 \mathrm{ixy} \Rightarrow(\overline{\mathrm{z}})^{2}+|\mathrm{z}|=0$

    \[ \Rightarrow x^{2}-y^{2}-2 i x y+\sqrt{x^{2}+y^{2}}=0 \Rightarrow x^{2}-y^{2}+\sqrt{x^{2}+y^{2}}-2 x y i=0 \Rightarrow x^{2}-y^{2}+\sqrt{x^{2}+y^{2}}=0 \text { and } x y=0 \]

    Case $\mathrm{I}: \mathrm{x}=0 \quad$ Case II : $\mathrm{y}=0$

    \[ \begin{array}{ll} -\mathrm{y}^{2}+|\mathrm{y}|=0 \Rightarrow|\mathrm{y}|=|\mathrm{y}|^{2} & \mathrm{x}^{2}+|\mathrm{x}|=0 \\ \Rightarrow|\mathrm{y}|(|\mathrm{y}|-1)=0 & \Rightarrow \mathrm{x}=0 \\ \Rightarrow \mathrm{y}=0, \pm 1 & \Rightarrow \mathrm{z}=0 \end{array} \]

    $\mathrm{z}=0, \mathrm{i},-\mathrm{i}$ are roots $\alpha=\mathrm{i}-\mathrm{i}=0 ; \beta=\mathrm{i}(-\mathrm{i})=1 \Rightarrow 4\left(\alpha^{2}+\beta^{2}\right)=4(0+1)=4$

    #11810
    Om Sharma
    Keymaster

    Answer

    C

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