The sum of all values of $\theta \, \in \,\left( {0,\frac{\pi }{2}} \right)$ satisfying ${\sin ^2}\,2\theta + {\cos ^4}\,2\theta = \frac{3}{4}$ is
$\pi $
$\frac{{5\pi }}{4}$
$\frac{{\pi }}{2}$
$\frac{{3\pi }}{8}$
$2\cos x - \cos 3x - \cos 5x = $
If $\tan \beta = \cos \theta \tan \alpha ,$ then ${\tan ^2}\frac{\theta }{2} = $
If $\cos \,(\theta - \alpha ) = a,\,\,\sin \,(\theta - \beta ) = b,\,\,$then ${\cos ^2}(\alpha - \beta ) + 2ab\,\sin \,(\alpha - \beta )$ is equal to
Prove that $\frac{\sin x-\sin 3 x}{\sin ^{2} x-\cos ^{2} x}=2 \sin x$
If $x = \cos 10^\circ \cos 20^\circ \cos 40^\circ ,$ then the value of $x$ is