Proving A Trigonometric Identity- Double Angles $(\cos(2x)-\sin(2x))(\sin(2x)+\cos(2x)) = \cos(4x)$ I'm trying to prove that the left side equals the right side. I'm just stuck on which double angle formula of cosine to use.

$cos(4x) = cos(2(2x)) = cos^2(2x)-sin^2(2x) = (cos(2x)-sin(2x))(cos(2x)+sin(2x))$
