sin^2a+sin^2b+2sina*sinb*cos(a+b)=sin^2a+sin^2b+2sina*sinb*(cosacosb-sinasinb)=(1-cos2a)/2+(1-cos2b)/2+sin2asin2b/2-(1-cos2a)(1-cos2b)/2=(1+sin2asin2b-cos2acos2b)/2