已知cos^2x-cos^2y=根号3/2,则sin(x+y)·sin(x-y)的值

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已知cos^2x-cos^2y=根号3/2,则sin(x+y)·sin(x-y)的值已知cos^2x-cos^2y=根号3/2,则sin(x+y)·sin(x-y)的值已知cos^2x-cos^2y=

已知cos^2x-cos^2y=根号3/2,则sin(x+y)·sin(x-y)的值
已知cos^2x-cos^2y=根号3/2,则sin(x+y)·sin(x-y)的值

已知cos^2x-cos^2y=根号3/2,则sin(x+y)·sin(x-y)的值
∵ cos^2x-cos^2y=(cosx-cosy)(cosx+cosy=)
= -2(sin(x/2-y/2))(sin(x/2+y/2))(cos(x/2-y/2))(cos(x/2+y/2))
= -sin(x-y)sin(x+y)=√3/2
∴sin(x-y)sin(x+y)= -√3/2
这里用到积化和差公式,这个没有正式提出,在习题中出现提到的