Trigonometric identity $2\cos(\alpha)\cos(\beta) = \cos(\alpha + \beta) + \cos(\beta - \alpha)$ in a book. Is it correct? The identity should be $2\cos(\alpha)\cos(\beta) = \cos(\alpha + \beta) + \cos(\alpha - \beta)$, but in the book (see attached picture) states it as follows
$2\cos(\alpha)\cos(\beta) = \cos(\alpha + \beta) + \cos(\beta - \alpha)$. What am I missing?


It's correct. $\cos x = \cos(-x) \forall x \in \mathbb R$ which means $\cos(A-B) = \cos (B-A) \forall A,B \in \mathbb R$
