唔該幫我證明呢條式···唔該曬！

cos²x - cos²y

= ( cosx - cosy ) ( cosx + cosy )

= ( - 2 sin(x + y)/2 sin(x - y)/2 ) ( 2 cos(x + y)/2 cos(x - y)/2 )

= - 2 sin(x + y)/2 cos(x + y)/2 * 2 sin(x - y)/2 cos(x - y)/2

= - sin(x + y) sin(x - y)

= ( - sinx cosy - cosx siny ) sin(x - y)

= ( sinx cosy - cosx siny - 2 sinx cosy ) sin(x - y)

= ( sin(x - y) - 2 sinx cosy ) sin(x - y)

= sin²(x - y) - 2 sinx cosy sin(x - y)

I'd prefer to start from RHS.