I derived the angle sum identity for cosine, namely, cos(a+b) = cos(a)cos(b)-sin(a)sin(b) and posted it on Mathematics Stack Exchange (the link has been provided at the end of this post). I think that the method may be pretty common but since I didn't find it anywhere, I thought of posting it. The final result of my derivation comes out to be cos(a+b) = cos(a)cos(b) ±sin(a)sin(b) and a comment suggested eliminating the ± symbol using an example where b = (-a). Here's the link to the post : https://math.stackexchange.com/questions/3704308/is-this-derivation-i-did-for-the-expansion-of-cos-alpha-beta-correct-and?noredirect=1#comment7612963_3704308 Let me know about your views on the derivation, thanks!