中四m2遇上難道,求解答!!!!!~

RHS
= sin^2 (x - y) - 2sinx cosy sin(x - y)
= sin(x - y)*[sin(x - y) - 2sinx cosy]
= (sinx cosy - cosx siny)*(sinx cosy - cosx siny - 2sinx cosy)
= -(sinx cosy - cosx siny)*(sinx cosy + cosx siny)
= cos^2 x sin^2 y - sin^2 x cos^2 y
= cos^2 x*(1 - cos^2 y) - (1 - cos^2 x)*cos^2 y
= cos^2 x - cos^2 x cos^2 y - cos^2 y + cos^2 x cos^2 y
= cos^2 x - cos^2 y
= LHS

圖片參考：http://imgcld.yimg.com/8/n/HA00016323/o/701203220055513873382190.jpg

http://i1099.photobucket.com/albums/g395/jasoncube/ee9199b3.png