Product to sum formula: sin α sin β
sin α sin β = ½[cos(α − β) − cos(α + β)]. Note that a product of two sines turns into cosines, and that the second term is subtracted - the one sign in the product-to-sum family worth double-checking.
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.