Sum to product formula: cos A + cos B

\cos A + \cos B = 2 \cos\tfrac{A+B}{2} \cos\tfrac{A-B}{2}

cos A + cos B = 2 cos((A + B)/2) cos((A − B)/2). Two cosines added give a product of two cosines, with no leading minus - which is what distinguishes it from the difference version.

Related Sum to Product identities

sin A + sin B sin A − sin B cos A − cos B

Practise it

Reading a formula is not the same as recalling it under time pressure. The workspace drills Sum to Product questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.

Practise Sum to Product Open the triangle solver

Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.