Prove

$\sin x \cdot \sin (\cos x)$

Let $\cos x=t$

$\therefore-\sin x d x=d t$

$\Rightarrow \int \sin x \cdot \sin (\cos x) d x=-\int \sin t d t$

$=-[-\cos t]+C$

$=\cos t+C$

$=\cos (\cos x)+C$