Ta có
$VT = \dfrac{\cos^2x - \sin^2x}{cot^2x - \tan^2x}$
$= \dfrac{\sin^2 x \cos^2 x(\cos^2x - \sin^2x)}{\cos^4x - \sin^4x}$
$= \dfrac{\sin^2x \cos^2x (\cos^2x - \sin^2x)}{(\cos^2x - \sin^2x)(\cos^2x + sin^2x)}$
$= \sin^2x \cos^2x$
$= \dfrac{1}{4} \sin^2(2x)$
$= \dfrac{1}{8} [1 - \cos(4x)] = VP$