sin(α+β)*sin(β-α)
=(sinαcosβ+cosαsinβ)(sinβcosα-cosβsinα)
=(sinβcosα)^2-(cosβsinα)^2
=[1-(cosβ)^2](cosα)^2-[1-(cosα)^2](cosβ)^2
=(cosα)^2-(cosβ)^2=m