sin^2a+sin^2b+2sinasinbcos（a+b）=sin^2a+sin^2b+2sinasinb(cosacosb-sinasinb)=(sin^2a-sin^2asin^2b)+(sin^2b-sin^2asin^2b)+2sinasinbcosacosb=sin^2acos^2b+sin^2bcos^2a+2sinasinbcosacosb=(sinacosb+sinbcos...