设sin(x+y)sin(x-y)=m,则cos^2x-cos^2y的值

sin(x+y)sin(x-y)=[sinxcosy+sinycosx][sinxcosy-cosxsiny]=(sinxcosy)^2-(cosxsiny)^2
=(1-cos^2y)cos^2y-cos^2x(1-cos^2y)
=cos^2y-cos^2x=m
故cos^2x-cos^2y=-m(sinxcosy)^2-(cosxsiny)^2怎么到(1-cos^2y)cos^2y-cos^2x(1-cos^2y)(sinxcosy)^2-(cosxsiny)^2 =sin^2xcos^2y-cos^2xsin^2y 不好意思 我(1-cos^2y)那里写错了 应该是(1-cos^2x) 如下=(1-cos^2x)cos^2y-cos^2x(1-cos^2y) =cos^2y-cos^2x