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Homework Help: Help Needed Solving an Equation

  1. Oct 17, 2009 #1
    1. The problem statement, all variables and given/known data
    This isn't really homework. I've just been practicing some equations and I'm stuck at the one below.

    2. Relevant equations
    (4/3)x^3 + x^2 - 2x + 8/3 = 0

    3. The attempt at a solution
    The quadratic formula does not apply to this so how do I solve it for x?

  2. jcsd
  3. Oct 17, 2009 #2
    The first thing I would do is multiply both sides by 3/4 to get rid of the ugly leading term. Then try using the rational roots theorem. In short, if there is a rational root for your polynomial, then it is given by a factor of the coefficient of x0 over a factor of the coefficient of x3. This will hopefully give you a very short list to try. If none of those work, then the polynomial has no rational roots and you have to use more advanced methods. If it does work, you can use long division to get the rest of the roots, if any.
    Last edited: Oct 17, 2009
  4. Oct 17, 2009 #3
    Thanks for the quick reply. I'll try that although its not something I'm familiar with.
  5. Oct 17, 2009 #4
    For cubics, there is always Cardano's formula, but it is rather messy in that it can give complicated complex expressions for simple real roots. The rational root method is often faster and less time consuming.
    Otherwise, as you are posting in the calculus forum, you can use Newton-Raphson to approximate a root if there are no rational roots.
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