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Polynomial Functions (word problem)

  1. Sep 29, 2011 #1
    1. The problem statement, all variables and given/known data
    A quartic function has x-intercepts of -5, -1, 2, and 4. The point (3.-8) lies on the curve. Determine an equation of the function.


    2. Relevant equations



    3. The attempt at a solution
    y = k (x+5)(x+1)(x-2)(x-4)
    Expanded form: f(x)=kx^4 + 9x^2 - 38x + 40 <---My expanded form is most likely wrong because expanding a quartic function tends to get clustered and confusing.

    Other than this I have absolutely no idea how to solve this question.
     
  2. jcsd
  3. Sep 29, 2011 #2

    gb7nash

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    This looks fine.

    Unfortunately, your expansion is wrong. First show your steps, so we can see where the mistake is in your work.
     
  4. Sep 29, 2011 #3

    eumyang

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    I wouldn't even expand yet. I would find k first by plugging 3 and -8 for x and y into the factored form. Then I would multiply out the factors, and distribute the k value last.
     
  5. Sep 30, 2011 #4

    gb7nash

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    Valid point. It's much easier to solve for k in this form.
     
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