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Math Help - Find the exact value

  1. #1
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    Find the exact value

    1)  \arccos ( \frac{-1}{2} )

    Attempt using the calculator

    I am getting 120 degrees but should I take 120 degrees or the reference angle which is 60 as the answer?
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  2. #2
    MHF Contributor
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    Hi

    0 \leq 120 \leq 180 therefore Arccos\left(-\frac12\right) = 120\: deg
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  3. #3
    A riddle wrapped in an enigma
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    Quote Originally Posted by mj.alawami View Post
    1)  \arccos ( \frac{-1}{2} )

    Attempt using the calculator

    I am getting 120 degrees but should I take 120 degrees or the reference angle which is 60 as the answer?
    Hi mj,

    Cosine is negative in QII and QIII. Cos 120 = -1/2.

    The angle in QIII would be Cos(240) = -1/2
    Last edited by masters; April 8th 2009 at 11:45 AM.
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