cosα = cos[(α + β) - β]
= cos(α + β)cosβ + sin(α + β)sinβ
= (cosαcosβ - sinαsinβ)cosβ + (sinαcosβ + cosαsinβ)sinβ
= cosαcos²β - sinαsinβcosβ + sinαsinβcosβ + cosαsin²β