左边=(sinacosβ+cosasinβ)·(cosacosβ+sinasinβ)
=sinacosa(cosβ)^2+sinacosa(sinβ)^2+sinβcosβ(sina)^2+sinβcosβ(cosa)^2
=sinacosa[(cosβ)^2+(sinβ)^2]+sinβcosβ[(sina)^2+(cosa)^2]
=sinacosa+sinβcosβ=右边