结果是1(sina)^2+(sinb)^2-(sina)^2*(sinb)^2+(cosa)^2*(cosb)^2=(sina)^2*(1-sinb^2)+(sinb)^2+(cosa)^2*(cosb)^2=sina^2*cosb^2+cosa^2*cosb^2+sinb^2=(sina^2+cosa^2)*cosb^2+sinb^2=cosb^2+sinb^2=1