Prove that $\frac{\cos (\pi+x) \cos (-x)}{\sin (\pi-x) \cos (\frac{\pi}{2}+x)} = \cot^{2} x$.

(N/A) We start with the $L.H.S.$: $\frac{\cos (\pi+x) \cos (-x)}{\sin (\pi-x) \cos (\frac{\pi}{2}+x)}$
Using the allied angle formulas:
$\cos (\pi+x) = -\cos x$
$\cos (-x) = \cos x$
$\sin (\pi-x) = \sin x$
$\cos (\frac{\pi}{2}+x) = -\sin x$
Substituting these values into the expression:
$= \frac{(-\cos x)(\cos x)}{(\sin x)(-\sin x)}$
$= \frac{-\cos^{2} x}{-\sin^{2} x}$
$= \frac{\cos^{2} x}{\sin^{2} x}$
$= \cot^{2} x = R.H.S.$