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Math Help - trigonometry

  1. #1
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    trigonometry

    hi!
    Can someone teach me how to solce arccos(cos(7Pi/ 6))?

    i tried to change 7pi/ 6 into the principal angle but what i thought of was:

    cos (7pi/6)= -cos (pi/6)

    although it is in the principal angle, i do not know how to continue after that.

    thanks!
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  2. #2
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    Quote Originally Posted by alexandrabel90 View Post
    hi!
    Can someone teach me how to solce arccos(cos(7Pi/ 6))?

    i tried to change 7pi/ 6 into the principal angle but what i thought of was:

    cos (7pi/6)= -cos (pi/6)

    although it is in the principal angle, i do not know how to continue after that.

    thanks!
    HI

    it would be \frac{5\pi}{6}

    cos (7pi/6)= -cos (pi/6)=cos (5pi/6)

    Lets say

    \cos^{-1} \cos (\frac{5\pi}{6})=x

    \cos (\frac{5\pi}{6})=\cos x

    So now , cancel the 'cos' , thus x=\frac{5\pi}{6}

    Note that its different when its \cos \cos^{-1} something
    Last edited by mathaddict; October 25th 2009 at 03:35 AM.
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  3. #3
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    but i thought that way that you did only applies to when the angle is in the principal range of 0 to 180 degrees?

    so i was trying to convert 7pi/6 into the principal angle of cos x where x is in the principal range so that i can use the method that you have just used..
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  4. #4
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    Quote Originally Posted by alexandrabel90 View Post
    but i thought that way that you did only applies to when the angle is in the principal range of 0 to 180 degrees?

    so i was trying to convert 7pi/6 into the principal angle of cos x where x is in the principal range so that i can use the method that you have just used..
    yeah you are right , i just edited .
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