If $\theta$ is in the interval $\left(0, \frac{\pi}{2}\right)$ satisfying the equation $\cos 2 \theta \cdot…

If $\theta$ is in the interval $\left(0, \frac{\pi}{2}\right)$ satisfying the equation $\cos 2 \theta \cdot \sec ^4 \theta+\sec ^2 \theta=0$, then $\sin ^2 \theta=$
  1. $\frac{1}{3}$
  2. $\frac{3}{4}$
  3. $\frac{1}{2}$
  4. $\frac{2}{3}$

Solution

$\begin{array}{lrl}\text {Given, } \cos 2 \theta \cdot \sec ^4 \theta+\sec ^2 \theta=0, \theta \in(0, \pi / 2) \\ \Rightarrow & \left(1-\tan ^2 \theta\right) \sec ^2 \theta+\sec ^2 \theta=0 \\ \Rightarrow & \sec ^2 \theta\left[1-\tan ^2 \theta+1\right]=0 \\ \Rightarrow & \tan ^2 \theta=2 \\ \Rightarrow & \sin ^2 \theta=2 / 3\end{array}$

Asked in: AP EAMCET 2018 (23 Apr Shift 2)

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