Product to sum formula: cos α cos β
cos α cos β = ½[cos(α − β) + cos(α + β)]. Both terms carry a plus sign here, unlike the sine-times-sine version. It comes straight from adding the cosine sum and difference formulas.
Related Product to Sum identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Product to Sum questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.