1 + cos 2x = 2 cos^2 x
sin 2x = 2sin x cos x
(1 + cos x)(1 - cos x ) = 1 - cos^2 x = sin^2 x
因此上式等于
(2*sinx*cosx*cosx*sinx) / 2 * cos^2 x * sin^2 x = 1