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Math Help - Proving trigonometry equation

  1. #1
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    Proving trigonometry equation

    Prove that \cos^4x- \sin^4x\equiv\cos^2x-sin^2x

    I have the trig rules but don't know what to do with them.

    Any help would be much appreciated
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  2. #2
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    Quote Originally Posted by greghunter View Post
    Prove that \cos^4x- \sin^4x\equiv\cos^2x-sin^2x
    a^4 - b^4=(a^2+b^2)(a^2-b^2)
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  3. #3
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    Quote Originally Posted by Plato View Post
    a^4 - b^4=(a^2+b^2)(a^2-b^2)

    Thanks, that's really helpful
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