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Math Help - Proving Trig Identities.

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    Proving Trig Identities.

    Prove that cos^-1(1/5)+cos^-1(5/7)+cos^-1(19/35) = Pie without the use of a calculator? How would I do this?
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    Quote Originally Posted by zweevu View Post
    Prove that cos^-1(1/5)+cos^-1(5/7)+cos^-1(19/35) = Pie without the use of a calculator? How would I do this?
    Several approaches are possible. One approach might be the following:

    Let \cos \alpha = \frac{1}{5}, \cos \beta = \frac{5}{7} and \cos \gamma = \frac{19}{35}.

    Now evaluate \cos (\alpha + \beta + \gamma) by applying the compound angle formula several times.

    Note: Find \sin \alpha, \sin \beta and \sin \gamma by using the known values for cos and applying the Pythagorean Identity.
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