sin(A+B) = sin A cos B + cos A sin B
sin(A−B) = sin A cos B − cos A sin B
cos(A+B) = cos A cos B − sin A sin B
cos(A−B) = cos A cos B + sin A sin B
tan(A+B) = (tan A + tan B) / (1 − tan A tan B)
tan(A−B) = (tan A − tan B) / (1 + tan A tan B)
This formulas all can be used to evaluate trigonometric functions, proving identities, solving trigonometric equations, solving a sum formula, and deriving reduction formulas. They all should be memorized and can easily be done so by memorizing them in groups based on sine, cosine, and tangent.
If you are interested in how these formulas were derived, you can check out the following page by clicking here showing the derivation of the Sum and Difference Formulas.
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