In a triangle with sides $a$, $b$, $c$ and angles $\alpha$, $\beta$, $a^2 - b^2 = bc$. Prove $2\beta = \alpha$.
I've been trying to solve this for a while, but can't come up with anything useful. I've tried using the sine and cosine rules, to no avail. It seems like it's extremely simple, but I'm missing something.