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Math Help - Can someone please tell me where im going wrong

  1. #1
    dno
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    Can someone please tell me where im going wrong

    i keep on getting the wrong answer but cant work out where im going wrong

    solve: between 0 and 360

    3cos2 = 2cos^2

    3(2cos^2 - 1) = 2cos^2

    6cos^2 - 3 = 2cos^2

    4cos^2 - 3 = 0

    cos = sqr 3/4
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  2. #2
    Flow Master
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    Quote Originally Posted by dno View Post
    i keep on getting the wrong answer but cant work out where im going wrong

    solve: between 0 and 360

    3cos2 = 2cos^2

    3(2cos^2 - 1) = 2cos^2

    6cos^2 - 3 = 2cos^2

    4cos^2 - 3 = 0

    cos = sqr 3/4
    Where is the unknown? What you've posted is mathemetically meaningless.

    I assume you mean 3 \cos (2{\color{red}x}) = 2 \cos^2 {\color{red}x}.

    In which case your last line should be \cos {\color{red}x} = {\color{red}\pm} \frac{\sqrt{3}}{2}, that is, \cos {\color{red}x} = \frac{\sqrt{3}}{2} and \cos x = {\color{red}-} \frac{\sqrt{3}}{2}.
    Last edited by mr fantastic; December 8th 2008 at 06:00 PM.
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  3. #3
    dno
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    Quote Originally Posted by mr fantastic View Post
    Where is the unknown? What you've posted is mathemetically meaningless.

    I assume you mean 3 \cos (2{\color{red}x}) = 2 \cos^2 {\color{red}x}.

    In which case your last line should be \cos {\color{red}x} = {\color{red}\pm} \frac{\sqrt{3}}{2}, that is, \cos {\color{red}x} = \frac{\sqrt{3}}{2} and \cos x = {\color{red}-} \frac{\sqrt{3}}{2}.

    couldnt be bothered to right out the fetas and it makes it more confusing


    the answers in the back of the book are 30, 150, 210, 330
    Last edited by mr fantastic; December 8th 2008 at 06:01 PM. Reason: Fixed the quote of my edit
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  4. #4
    Flow Master
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    Quote Originally Posted by dno View Post
    couldnt be bothered to right out the fetas and it makes it more confusing


    the answers in the back of the book are 30, 150, 210, 330
    I fixed some typos in my previous reply. Go back and look at the final equations you have to solve. They should be simple to solve.
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  5. #5
    dno
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    Quote Originally Posted by mr fantastic View Post
    I fixed some typos in my previous reply. Go back and look at the final equations you have to solve. They should be simple to solve.
    oh yeah i get it now tanks
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