If $\tan ^{-1}\left(\frac{1-x}{1+x}\right)=\frac{1}{2} \tan ^{-1} x$, then $x$ has the value

If $\tan ^{-1}\left(\frac{1-x}{1+x}\right)=\frac{1}{2} \tan ^{-1} x$, then $x$ has the value
  1. $1$
  2. $\sqrt{3}$
  3. $3$
  4. $\frac{1}{\sqrt{3}}$

Solution

$\begin{aligned} & \tan ^{-1}\left(\frac{1-x}{1+x}\right)=\frac{1}{2} \tan ^{-1} x \\ & \Rightarrow \tan ^{-1}\left(\frac{1-\tan \theta}{1+\tan \theta}\right)=\frac{1}{2} \tan ^{-1}(\tan \theta) \\ & \Rightarrow \tan ^{-1}\left(\tan \left(\frac{\pi}{4}-\theta\right)\right)=\frac{1}{2} \theta \\ & \Rightarrow \frac{\pi}{4}-\theta=\frac{1}{2} \theta \\ & \Rightarrow \frac{\pi}{4}=\frac{3 \theta}{2} \\ & \Rightarrow \theta=\frac{\pi}{6} \\ & \Rightarrow x=\tan \frac{\pi}{6}=\frac{1}{\sqrt{3}}\end{aligned}$

Asked in: MHT CET 2022 (06 Aug Shift 2)

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