sin · cossinαcosβ=12[sin(α+β)+sin(α−β)]\sin\alpha \cos\beta = \tfrac{1}{2}[\sin(\alpha+\beta) + \sin(\alpha-\beta)]sinαcosβ=21[sin(α+β)+sin(α−β)]Open identity
cos · coscosαcosβ=12[cos(α−β)+cos(α+β)]\cos\alpha \cos\beta = \tfrac{1}{2}[\cos(\alpha-\beta) + \cos(\alpha+\beta)]cosαcosβ=21[cos(α−β)+cos(α+β)]Open identity
sin · sinsinαsinβ=12[cos(α−β)−cos(α+β)]\sin\alpha \sin\beta = \tfrac{1}{2}[\cos(\alpha-\beta) - \cos(\alpha+\beta)]sinαsinβ=21[cos(α−β)−cos(α+β)]Open identity