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Math Help - Cosine Problem

  1. #1
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    [SOLVED] Cosine Problem

    Find the small positive number x such that

    cos(x^2+2x+9)=0

    Round your answer to 4 decimal places.

    So far I've tried this equation: x^2+2x+(9-(2n+1)pi/2)=0
    put it into quadratic and for that i got x=-4.8688x10^-4 which wasn't correct

    Also tried: x^2+2x+(9-pi/2)=0
    got 1.9033 which also wasn't correct

    T_T;; please help I only have 1 more chance or I won't get full credit. I am totally lost, I have NO IDEA what to do...but I did try.
    Last edited by Chuka; May 10th 2012 at 01:21 AM. Reason: SOLVED
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  2. #2
    Member Goku's Avatar
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    Re: Cosine Problem

    Quote Originally Posted by Chuka View Post

    Also tried: x^2+2x+(9-pi/2)=0
    got 1.9033 which also wasn't correct
    .
    How did you get 1.9033,
    solutions to the above equation is :
    x1 = -1 + 2.5356i
    x2 = -1 + 2.5356 i

    In other words these give you no solutions,

    Try to solve this quadratic equation:
    x^2+2x+(9-7pi/2)=0 // cos((7 * pi) / 2) = 0

    and remember that for the quadratic equation to give you real values:

    (2n+1)pi/2 > 9 // solve for what values of n you get real values....
    Thanks from Chuka
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  3. #3
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    Re: Cosine Problem

    OMG THANK YOU!!!!!!!! Thank god XD, got it on my last chance.
    I actually did try 7pi/2 before but I made errors. Fail T_T..
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