Sum to product formula: cos A − cos B
cos A − cos B = −2 sin((A + B)/2) sin((A − B)/2). The leading minus sign is the one people drop: a difference of cosines becomes a negative product of sines, not a positive one.
Related Sum to Product identities
Common questions
What is the cos A − cos B identity?
cos A − cos B = −2 sin((A + B)/2) sin((A − B)/2). The leading minus sign is the one people drop: a difference of cosines becomes a negative product of sines, not a positive one.
Which identities are related to cos A − cos B?
cos A − cos B belongs to the Sum to Product group. The identities used alongside it most often are sin A + sin B, sin A − sin B and 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.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.