Here f(x)=sin x+cos xsin x−cos x
f′(x)=ddx[sin x+cos xsin x−cos x]
=(sin x−cos x)ddx(sin x+cos x)−(sin x+cos x)ddx(sin x−cos x)(sin x−cos x)2
=(sin x−cos x)(cos x−sin x)−(sin x+cos x)(cos x+sin x)(sin x−cos x)2
=−(sin x−cos x)2−(sin x+cos x)2(sin x−cos x)2
=−2(sin2x+cos2x)(sin x−cos x)2
=−2(sin x−cos x)2