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Math Help - extra credit

  1. #1
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    Cool extra credit

    For extra credit I have to show that:

    -5/39 sin8x sin5x - 8/39 cos8x cos5x = -cos13x/26 - cos3x/6.

    Any suggestions on where to start? Thanks for the help.
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  2. #2
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    Quote Originally Posted by Carol View Post
    For extra credit I have to show that:

    -5/39 sin8x sin5x - 8/39 cos8x cos5x = -cos13x/26 - cos3x/6.

    Any suggestions on where to start? Thanks for the help.
    From the compound angle formulae:

    \sin x \sin y = \frac{\cos(x - y) - \cos (x + y)}{2}

    \cos x \cos y = \frac{\cos(x - y) + \cos (x + y)}{2}
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