Product to sum formula: sin α cos β
sin α cos β = ½[sin(α + β) + sin(α − β)]. Products of trig functions are awkward to integrate and to graph; sums are not. This is also the identity behind acoustic beats, where two close frequencies multiply into a slow envelope.
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.