# Trigonometric Identity

• November 9th 2012, 08:04 AM
Greymalkin
Trigonometric Identity
Prove: $cos^2(2x)=\frac{cos4x+1}{2}$
• November 9th 2012, 08:56 AM
hedi
Re: Trigonometric Identity
use the identity for cos(2y) with y=2x.
• November 9th 2012, 09:07 AM
Greymalkin
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.
• November 9th 2012, 09:15 AM
hedi
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.