Christina DanielThis post includes proofs of two angle addition identities.
$ \sin(x+y) = \sin x \cos y + \sin y \cos x$
$ \cos(x+y) = \cos x \cos y – \sin x \sin y$
References
[1] https://www.khanacademy.org/math/precalculus/x9e81a4f98389efdf:trig/x9e81a4f98389efdf:angle-addition/v/proof-angle-addition-sine [2] https://www.khanacademy.org/math/precalculus/x9e81a4f98389efdf:trig/x9e81a4f98389efdf:angle-addition/v/proof-angle-addition-cosine