Câu 1.$VT=\frac{cosx-sinx}{cosx+sinx}=\frac{\frac{\sqrt{2}}{2}cosx-\frac{\sqrt{2}}{2}sinx}{\frac{\sqrt{2}}{2}cosx+\frac{\sqrt{2}}{2}sinx}$
$=\frac{sin\frac{\pi}{4}.cosx-cos\frac{\pi}{4}.sinx}{cos\frac{\pi}{4}.cosx+sin\frac{\pi}{4}.sinx}$
$=\frac{sin(\frac{\pi}{4}-x)}{cos(\frac{\pi}{4}-x)}=tan(\frac{\pi}{4}-x)=VP$(đpcm)
Câu 2.
$C=\frac{-(2cos^22x-cos2x-1)}{sin2x+2sin2x.cos2x}=\frac{-(cos2x-1)(1+2cos2x)}{sin2x(1+2cos2x)}=\frac{1-cos2x}{sin2x}$
$=\frac{2sin^2x}{2sinx.cosx}=\frac{sinx}{cosx}=tanx$