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Solving a cubic function

  1. Jun 19, 2012 #1
    1. The problem statement, all variables and given/known data

    Actually, part of a question on modeling/control systems. I need to find the poles (or, roots) of the cubic function.

    2. Relevant equations

    x^3 - 13x + 12 = 0

    3. The attempt at a solution

    The rule of thumb for the course is that if you get a cubic function then there will always be a solution of x equal to -1,0 or 1. For this particular equation, I figured out that it's 1. Now I need 2 more solutions.

    On my first try I got +/- sqrt(13) by factoring the equation as follows:

    x(x^2-13x)+12=0

    Those are wrong, I checked them. So yeah, question is if someone can help me or point me in the direction of a simple way to solve cubic functions, I have almost no experience with them (normally I'd use my calculator, not allowed for this class).

    The answers in the appendix say that the other two roots are 3 and -4, if that helps.

    Thanks in advance!
     
  2. jcsd
  3. Jun 19, 2012 #2

    hotvette

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    Homework Helper

    Since you know the first root is 1, you can write:

    (x-1)(x+a)(x+b) = x3-13x+12

    From this you can tell that -1*a*b = 12, thus a*b = -12. Additionally, since the x2 term is zero, you can multiply the left side and set the x2 term to zero, revealing that a + b = 1. The only possibility where a*b = -12 and a + b = 1 is a = 4 and b = -3, which means x = -4 and x = 3 are roots.
     
    Last edited: Jun 19, 2012
  4. Jun 20, 2012 #3
    Thanks mate, actually don't know why I didn't get this immediately :)
     
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