Sine sum and difference formula: sin(α ± β) = sin α cos β ± cos α sin β
sin(α ± β) = sin α cos β ± cos α sin β. The signs match: a plus inside gives a plus outside. Use it to get exact values for angles you can build from known ones - sin 75° = sin(45° + 30°) - and to derive the double-angle formula by setting β = α.
Related Sum & Difference identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Sum & Difference 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.