求证:cosθ-cosφ=-2sinθ+φ/2sinθ-φ/2
问题描述：
求证:cosθ-cosφ=-2sinθ+φ/2sinθ-φ/2
答
可怜的孩子!你应该加括号的.是 cos θ -cos φ = -2 sin [ (θ+φ)/2 ] sin [ (θ-φ)/2 ] = = = = = = = = =令 x =(θ+φ)/2,y = (θ-φ)/2,则 θ =x +y,φ =x -y.所以 cos θ =cos (x +y)=cos x cos y -sin x sin ...