求证sin^2x+sin^2y-sin^2x*sin^2y+cos^2x*cos^2y=1
问题描述：
求证sin^2x+sin^2y-sin^2x*sin^2y+cos^2x*cos^2y=1
答
sin^2x+sin^2y-sin^2x*sin^2y+cos^2x*cos^2y= sin^2x-sin^2x*sin^2y+sin^2y+cos^2x*cos^2y= sin^2x*(1-sin^2y)+sin^2y+cos^2x*cos^2y= sin^2x*cos^2y+sin^2y+cos^2x*cos^2y= sin^2x*cos^2y+cos^2x*cos^2y+sin^2y= co...