(b)
We have, $y = \log_{\sin x}{\cos x}$
$= \frac{\log{\cos x}}{\log{\sin x}}$
$= \frac{\log{\sin{x\frac{1}{\cos x}( - \sin{x) - \log{\cos{x\frac{1}{\sin x}\cos x}}}}}}{\left( \log{\sin x} \right)^{2}}$
$= \frac{- \tan{x\log{\sin{x - \cot{x\log{\cos x}}}}}}{\left( \log{\sin x} \right)^{2}}$