[sin^2a-sin^2b]/sinacosa-sinbcosb=[sin^2-sin^2b]/[(1/2)sina-(1/2)sinb]=2(sina+sinb)(sina-sinb)/(sina-sinb)=2(sina+sinb).
=sin(a+b)sin(a-b)/[sin(a+b)cos(a-b)]=tg(a+b)
