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Math Help - Express cos^2 (2x) in terms of cos 4x

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    Question 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.
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  2. #2
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    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*}.
    Last edited by Prove It; May 13th 2012 at 06:00 AM.
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    Re: Express cos^2 (2x) in terms of cos 4x

    Is it + 1 or -1 at the end?
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    Re: Express cos^2 (2x) in terms of cos 4x

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