If $\cos x=\tan y, \cot y=\tan z$ and $\cot z=\tan x$, then $\sin x$ equals to

If $\cos x=\tan y, \cot y=\tan z$ and $\cot z=\tan x$, then $\sin x$ equals to
  1. $\frac{\sqrt{5}+1}{4}$
  2. $\frac{\sqrt{5}-1}{4}$
  3. $\frac{\sqrt{5}+1}{2}$
  4. $\frac{\sqrt{5}-1}{2}$

Solution

Given, $\cos x=\tan y, \cot y=\tan z$ and $\cot z=\tan x$ $\therefore \quad \cos x=\tan y$ $\Rightarrow \quad \cos x=\frac{1}{\tan z}$ $\Rightarrow \quad \cos x=\cot z$ $\Rightarrow \quad \cos x=\tan x$ $\Rightarrow \quad \cos x=\frac{\sin x}{\cos x}$ $\Rightarrow \cos ^2 x=\sin x$ $\Rightarrow 1-\sin ^2 x=\sin x$ $\Rightarrow \sin ^2 x+\sin x-1=0$ $\begin{aligned} \therefore \quad \sin x & =\frac{-1 \pm \sqrt{1-4 \times(-1)}}{2 \times 1} \\ & =\frac{-1 \pm \sqrt{5}}{2} \\ \therefore \sin x=\frac{\sqrt{5}-1}{2} & \left(\because \frac{-1-\sqrt{5}}{2} < -1\right)\end{aligned}$

Asked in: AP EAMCET 2014

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