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Math Help - Theorem

  1. #1
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    Theorem

    Use Theorem s(x+y)= s(x)c(y)+c(x)s(y) to show

    cos(4x)=

    Then apply this formula to find the value of cos(/8)
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  2. #2
    MHF Contributor MarkFL's Avatar
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    Re: Theorem

    \cos(4x)=\cos(2(2x))=1-2\sin^2(2x)=1-2(\sin(x+x))^2

    Now apply the angle-sum identity for sine as required.

    Then use:

    x=\frac{\pi}{8} in the resulting formula. I would suggest letting u=\cos^2(x) to get a quadratic in u. Take the appropriate root.
    Last edited by MarkFL; November 13th 2012 at 10:00 PM.
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