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Math Help - Exact value for cos 13pi/12

  1. #1
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    Exact value for cos 13pi/12

    SOLVED IT. Sorry.

    Question:

    Determine an exact value for cos 13pi/12.

    Since 13pi/12 is a degree of 195 degrees which is not related to a right angle triangle, I tried breaking it up into 3pi/4 + pi/3. Then I found the cos of both of those which got me:

    cos(13pi/12) = cos(3pi/4) + cos(pi/3)
    = (-1/sqr2) + (1/2)

    I have no clue if i'm on the right track..

    The answer: -1-sqr3 / 2sqr2
    Last edited by kmjt; November 26th 2009 at 11:30 AM.
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  2. #2
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    Quote Originally Posted by kmjt View Post

    cos(13pi/12) = cos(3pi/4) + cos(pi/3)
    Are you sure this is correct?

    I believe

    \cos\left(\frac{13\pi}{12}\right) = \cos\left(\frac{3\pi}{4}+\frac{\pi}{3}\right) \neq \cos\left(\frac{3\pi}{4}\right)+\cos\left(\frac{\p  i}{3}\right)
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  3. #3
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    Yes your right what I have in first post is wrong hehe. But I solved it using a compound angle formula.
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