∵ cos^2x-cos^2y=（cosx-cosy）（cosx+cosy=）= -2（sin（x/2-y/2）)(sin（x/2+y/2）)（cos(x/2-y/2))(cos(x/2+y/2）)= -sin
(x-y)sin(x+y)=√3/2∴sin(x-y)sin(x+y) = -√3/2