sin(x+y)sin(x-y)=k,求cos^2x-cos^2y
问题描述：
sin(x+y)sin(x-y)=k,求cos^2x-cos^2y
答
-2k=cos2x-cos2y
=[2(cosx)^2-1]-[2(cosy)^2-1]
=2[(cosx)^2-(cosy)^2]
cos^2x-cos^2y=-k