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Math Help - Solving Trig Function Problem "by hand" (given values, no calculator)

  1. #1
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    Unhappy Solving Trig Function Problem "by hand" (given values, no calculator)

    Given sin( π/5 ) = 0.588 and cos ( π/8 ) = 0.924

    How can you figure out cos ( 27π/8 )?

    I can't think of any trig identities/formulas that would fit the problem. Help please?

    Thanks in advance.

    (π is pi btw)
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  2. #2
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    I can't straight away see how this one can be solved.

    But I would suggest having a look at the indentities for \cos(a+b) and \cos(a-b)
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  3. #3
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    ohh I got it now. I never thought about the addition identities! I got the right answer now. Thanks!
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