左边 1/2(1+sina-cosa)的平方
= 1/2(1+sin^2a+cos^2a+2sina-2cosa-2sinacosa)
= 1/2(1+1+2sina-2cosa-2sinacosa)
= 1/2(2+2sina-2cosa-2sinacosa)
= 1+sina-cosa-sinacosa
= (1+sina)(1-cosa)