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

1. ## 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.

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

Do you know that \displaystyle \displaystyle \begin{align*} \cos{2\theta} &\equiv 2\cos^2{\theta} - 1 \end{align*}? Let \displaystyle \displaystyle \begin{align*} \theta = 2x \end{align*}.

3. ## Re: Express cos^2 (2x) in terms of cos 4x

Is it + 1 or -1 at the end?

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

Originally Posted by johnreal
Is it + 1 or -1 at the end?
Sorry it's -1. Editing...

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### express cos^2 2x in terms of cos 4x

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