Talk:Cosine of Sum

Domain
The domain is never specified. If the domain is $\R$, then the first proof which invokes complex numbers is invalid. If the domain is $\C$, then it is next to impossible to identify the real parts, since $\cos a$ might be complex. --kc_kennylau (talk) 11:51, 28 October 2016 (EDT)


 * I accept your point about the complex case, but why is the proof invalid if the domain is real? --prime mover (talk) 14:31, 28 October 2016 (EDT)


 * I take back my point. The domain is $\R$ then. --kc_kennylau (talk) 20:03, 28 October 2016 (EDT)


 * So, all proofs except the third one apply only to $\R$. Should I tag it as refactor? --kc_kennylau (talk) 08:24, 29 October 2016 (EDT)