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Math Help - x^3 - 6x^2 + 13x - 12 = 0

  1. #1
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    x^3 - 6x^2 + 13x - 12 = 0

    x^3 - 6x^2 + 13x - 12 = 0-untitled2.png

    The method is than involving the formula (1/2)(-k+srt(k^2 +4h^3)) and the book is Liebeck's "A Concise Introduction to Pure Mathematics". Hopefully that is familiar to someone.

    My attempt:

    x^3 - 6x + 13x - 12 = 0

    Let y = x-2

    So y^3 = x^3 - 6x + 12x - 8
    So y^3 + x - 4 = 0
    So y^3 + y - 2 = 0

    h = 1/3 , k = -2

    Pluging into the formula gives y = crt((9+2srt(2))/9) + crt((9-2srt(2))/9)

    From here I am totally stuck? Is this even right so far?

    thanks
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    Quote Originally Posted by kinhew93 View Post
    Click image for larger version. 

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    Note that (x-3) is a factor. Divide and get a quadratic that you can solve.
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    Hey kinhew93.

    In terms of reconciling the roots, note that if 3 is a root then you can factor your polynomial by dividing by (x-3) and you should have no remainder. In other words:

    P(x) = x^3 - 6x^2 + 13x - 12 = (x-3)(ax^2 + bx + c). You can use this to find a, b, and c and then use the quadratic formula to verify your other roots.

    This is in addition to the cubic formula (which I haven't seen for a long time since first year university) and you can use the above to check your answer.
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    Quote Originally Posted by Plato View Post
    Note that (x-3) is a factor. Divide and get a quadratic that you can solve.
    I know that but the point of the question is that it is asking to use a specific method (one that involves (1/2)(-k+srt(k^2 +4h^3)) )

    ?
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    Quote Originally Posted by kinhew93 View Post
    I know that but the point of the question is that it is asking to use a specific method (one that involves (1/2)(-k+srt(k^2 +4h^3)) )
    Then that is a nonsense question. Why are you in such an outdated course?
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    Quote Originally Posted by Plato View Post
    Then that is a nonsense question. Why are you in such an outdated course?
    So you don't think there's any value in solving problems in different ways?
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    The specific method you are asked to use looks a lot like the equation numbered (20) on this page Cubic Formula -- from Wolfram MathWorld
    The notation they use is a little different though
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  8. #8
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    Re: x^3 - 6x^2 + 13x - 12 = 0

    The method is known as Vieta's method.

    (Francois Viete (aka Franciscus Vieta) 1540-1603)

    Anyone interested in the history and development of mathematics should find it interesting.

    The first step, to eliminate the squared term, has been completed (successfully).

    Next is to use a second substitution, y = w - 1/(3w) and that will get you a quadratic in w cubed.

    Solve that using the usual formula for a quadratic and you have two values for w cubed.

    Calculate w (from either value, it doesn't matter which, they give you the same value for y,) and back substitute for y and then for x.

    The other two roots, which could be complex, can be calculated either by factoring out the solution just calculated and then solving the resulting quadratic, or by finding the two other (complex) values of w and then again back substituting twice.
    Thanks from kinhew93
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