Giải thích các bước giải:
$VT=\dfrac{ 1-\sin^2x\cos^2x}{\cos^2x}-\cos^2x\\
=\dfrac{ \sin^2x+\cos^2x-\sin^2x\cos^2x}{\cos^2x}-\cos^2x\\
=\dfrac{ \sin^2x+\cos^2x(1-\sin^2x)}{\cos^2x}-\cos^2x\\
=\dfrac{ \sin^2x+\cos^4x}{\cos^2x}-\cos^2x\\
=\tan^2x+\cos^2x-\cos^2x\\
=\tan^2x=VP\Rightarrow ĐPCM$