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

**johnreal** · May 13th 2012, 05:18 AM

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.

**Prove It** · May 13th 2012, 05:20 AM

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

**johnreal** · May 13th 2012, 05:27 AM

Is it + 1 or -1 at the end?

**Prove It** · May 13th 2012, 06:00 AM

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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