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Math Help - Prove: Double angles

  1. #1
    Newbie
    Joined
    May 2010
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    Prove: Double angles

    Because our teacher made a test hard enough that no one in our class understood, he decided to give it to us as open-book homework. I really need help with some questions.

    Prove:
    1 - tan^2x = cos2x
    sec^2x

    In case anyone doesn't know what I mean, ^2 means squared.

    Thanks to whoever can help
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  2. #2
    MHF Contributor
    Joined
    Nov 2008
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    France
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    1,458
    Hi

    You should try by your own if you want to improve

    \frac{1-\tan^2x}{\sec^2x} = \frac{1}{\sec^2x}-\frac{\tan^2x}{\sec^2x} = \cos^2x - \sin^2x = \cos(2x)
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  3. #3
    Newbie
    Joined
    May 2010
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    I tried by myself for a couple hours, so I was kind of just wasting time. Thank you so much for helping me, I actually understand this one now
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