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Math Help - sums and differences of angles

  1. #1
    Junior Member
    Joined
    Sep 2009
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    sums and differences of angles

    If the angles A, B of triangle ABC are such that cos A = {-4/5} and cos B = {12/13}, find without using tables the value of Cos C'

    Thanks.
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  2. #2
    MHF Contributor
    Joined
    Apr 2008
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    C = \pi - A - B

    \cos C = \cos(\pi - A - B)

    Using identities:

    \cos C = \cos(\pi - A)\cos B + \sin(\pi - A)\sin B

    \cos C = -\cos A \cos B + \sin A \sin B

    You should be able to calculate \sin A and \sin B.
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