# Express cos^2 (2x) in terms of cos 4x

• May 13th 2012, 05:18 AM
johnreal
Express cos^2 (2x) in terms of cos 4x
I know i'm supposed to use double angle formula, but i still don't get how to get my answer. I am to Express cos^2 (2x) in terms of cos 4x.(Crying)
• May 13th 2012, 05:20 AM
Prove It
Re: Express cos^2 (2x) in terms of cos 4x
Do you know that \displaystyle \begin{align*} \cos{2\theta} &\equiv 2\cos^2{\theta} - 1 \end{align*}? Let \displaystyle \begin{align*} \theta = 2x \end{align*}.
• May 13th 2012, 05:27 AM
johnreal
Re: Express cos^2 (2x) in terms of cos 4x
Is it + 1 or -1 at the end?
• May 13th 2012, 06:00 AM
Prove It
Re: Express cos^2 (2x) in terms of cos 4x
Quote:

Originally Posted by johnreal
Is it + 1 or -1 at the end?

Sorry it's -1. Editing...