# Math Help - Trigonometric Identity

1. ## Trigonometric Identity

Prove: $cos^2(2x)=\frac{cos4x+1}{2}$

2. ## Re: Trigonometric Identity

use the identity for cos(2y) with y=2x.

3. ## Re: Trigonometric Identity

please elaborate, I dont think my trig book explained double angles beyond the basic identities, all I can think of doing is multiplying double id's together but I dont see how you double a double angle, that is to say make 2x=4x.

4. ## Re: Trigonometric Identity

cos(4x)=2cos^2(2x)-1 comes from cos(2x)=2cos^2(x)-1 when substituting 2x for x in the later equation.