4-1.Complex numbers
hard

माना $z _{1}$ तथा $z _{2}$ कोई दो शून्येतर सम्मिश्र संख्याएँ इस प्रकार हैं कि $3\left| z _{1}\right|=4\left| z _{2}\right|$ है। यदि $z =\frac{3 z _{1}}{2 z _{2}}+\frac{2 z _{2}}{3 z _{1}}$ हो, तो

A

Re$(z) = 0$

B

$\left| z \right| = \sqrt {\frac{5}{2}} $

C

$\left| z \right| = \frac{1}{2}\sqrt {\frac{{17}}{2}} $

D

Im$(z) \neq 0$

(JEE MAIN-2019)

Solution

$\left|\frac{3 z_{1}}{2 z_{2}}\right|=2$

Let $\frac{3 z_{1}}{2 z_{2}}=2 \cos \theta+2(\sin \theta) i$

$\Rightarrow \frac{2 z_{2}}{3 z_{1}}=\frac{1}{2} \cos \theta-\frac{1}{2}(\sin \theta)$

Given, $z=\frac{2 z_{1}}{3 z_{2}}+\frac{3 z_{2}}{2 z_{1}}$ $=\frac{5}{2} \cos \theta+\frac{3}{2}(\sin \theta) i$

Which is neither purely real nor purely imaginary and $|z|$ depends on $\theta$.

Standard 11
Mathematics

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