This is question 3 from the APICS Mathematics Competition paper of 1999:
Prove that $$\sin^2(x+\alpha)+\sin^2(x+\beta)-2\cos(\alpha-\beta)\sin(x+\alpha)\sin(x+\beta)$$ is a constant function of $x$.
Expanding it seems rather daunting, in particular the last term, and nothing I've tried has been useful towards cancelling terms out. It was assigned in a pre-calculus course, so it should be possible to solve without using derivatives. However, showing that $f'(x)=0$ would obviously be a valid solution.
Any ideas are greatly welcome.