Ta có: $VT=\frac{2cos2x-2sin2x.cos2x}{2cos2x+2sin2x.cos2x}=\frac{2cos2x(1-sin2x)}{2cos2x(1+sin2x)}$$=(\frac{cosx-sinx}{cosx+sinx})^2=(\frac{\sqrt{2}sin(\frac{\pi}{4}-x)}{\sqrt{2}cos(\frac{\pi}{4}-x)})^2=\frac{sin^2(\frac{\pi}{4}-x)}{cos^2(\frac{\pi}{4}-x)}=tan^2(\frac{\pi}{4}-x)=VP$ (đpcm)