Let $f(x) = \cos \sqrt {x,} $ then which of the following is true
$f(x)$ is periodic with period $\sqrt 2 \pi $
$f(x)$ is periodic with period $\sqrt \pi $
$f(x)$ is periodic with period $4{\pi ^2}$
$f(x)$ is not a periodic function
If $\theta $ and $\phi $ are acute satisfying $\sin \theta = \frac{1}{2},$ $\cos \phi = \frac{1}{3},$ then $\theta + \phi \in $
The number of solutions that the equation $sin5\theta cos3\theta = sin9\theta cos7\theta $ has in $\left[ {0,\frac{\pi }{4}} \right]$ is
If $\sqrt 2 \sec \theta + \tan \theta = 1,$ then the general value $\theta $ is
The number of solutions of the equation $4 \sin ^2 x-4$ $\cos ^3 \mathrm{x}+9-4 \cos \mathrm{x}=0 ; \mathrm{x} \in[-2 \pi, 2 \pi]$ is :
$\tan \,{20^o}\cot \,{10^o}\cot \,{50^o}$ is equal to