证明sin(x+y)sin(x-y)=sinx-siny
问题描述：
证明sin(x+y)sin(x-y)=sinx-siny
答
sin(x+y)sin(x-y) =-1/2(cos(x+y+x-y)—cos(x+y-x+y)) =-1/2(cos2x—cos2y) =-1/2(1-2（sinx）^2-1+2(siny）^2) =（sinx）^2-(siny）^2