I strongly recommend looking at the geometric proof of the angle sums formulae for sine and cosine. https://en.wikipedia.org/wiki/List_of_trigonometric_identit

I strongly recommend looking at the geometric proof of the angle sums formulae for sine and cosine. https://en.wikipedia.org/wiki/List_of_trigonometric_identities#/media/File:AngleAdditionDiagramSine.svg https://en.wikipedia.org/wiki/List_of_trigonometric_identiti...