This works well if you're in a situation where you need to have rotation matrices memorized. If you're not (and many people first learning trigonometry are not), then remembering the standard double angle formulas (i.e. cos( 2x )=cos^2( x )-sin^2( x ), etc.) , combined with the symmetry of A and B is useful. As far as remembering the double angle formulae goes, I don't think I know a good method.