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JEE Advanced 2022 · Complex Numbers and Quadratic Equations

Let zz be a complex number with non-zero imaginary part. If 2+3z+4z22−3z+4z2\frac{2+3z+4z^{2}}{2-3z+4z^{2}} is a real number, then the value of ∣z∣2|z|^{2} is ______.

Answer: 0.500.50

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Written solution

Given: z∈C, Im⁡(z)≠0\text{Given: } z \in \mathbb{C}, \ \operatorname{Im}(z) \neq 0 2+3z+4z22−3z+4z2∈R\dfrac{2+3z+4z^{2}}{2-3z+4z^{2}} \in \mathbb{R} To find: ∣z∣2\text{To find: } |z|^{2} w=2+3z+4z22−3z+4z2w = \dfrac{2+3z+4z^{2}}{2-3z+4z^{2}} A=2+4z2,B=3zA = 2+4z^{2}, \quad B = 3z w=A+BA−Bw = \dfrac{A+B}{A-B} w=wˉw = \bar w A+BA−B=Aˉ+BˉAˉ−Bˉ\dfrac{A+B}{A-B} = \dfrac{\bar A+\bar B}{\bar A-\bar B} (A+B)(Aˉ−Bˉ)=(Aˉ+Bˉ)(A−B)(A+B)(\bar A-\bar B) = (\bar A+\bar B)(A-B) AAˉ−ABˉ+BAˉ−BBˉA\bar A - A\bar B + B\bar A - B\bar B =AˉA−AˉB+BˉA−BˉB= \bar A A - \bar A B + \bar B A - \bar B B BAˉ=ABˉB\bar A = A\bar B 3z (2+4zˉ2)=(2+4z2) 3zˉ3z\,(2+4\bar z^{2}) = (2+4z^{2})\,3\bar z 2z+4zzˉ2=2zˉ+4z2zˉ2z + 4z\bar z^{2} = 2\bar z + 4z^{2}\bar z 2(z−zˉ)=4zzˉ (z−zˉ)2(z-\bar z) = 4z\bar z\,(z-\bar z) Im⁡(z)≠0⇒z≠zˉ\operatorname{Im}(z) \neq 0 \Rightarrow z \neq \bar z 1=2 zzˉ=2∣z∣21 = 2\,z\bar z = 2|z|^{2} ∣z∣2=12|z|^{2} = \dfrac{1}{2} z=12+12i⇒∣z∣2=12  ✓z = \tfrac{1}{2}+\tfrac{1}{2}i \Rightarrow |z|^{2} = \tfrac{1}{2} \; \checkmark ∣z∣2=12\boxed{|z|^{2} = \tfrac{1}{2}}

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