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Homework Help: Calculus problems (hmmt)

  1. Nov 25, 2008 #1
    The question is :"The polynomial 3x5-250x3+735x is interesting because it has the maximum possible number of relative extrema and points of inflection at integer lattice points for a quintic polynomial. What is the sum of the x-coordinates of these points?"

    I think the answer is 0, but it's wrong.

    My solution is :"The first derivative is 15x4-750x2+735, whose roots are -7,-1,1 and 7. The second derivative is 60x3-1500x,whose roots are 0,-5,5. Then, the sum is 0."

    The HMMT solution is "The first derivative is 15x4-750x2+735, whose roots sum to 750/15=50. The second derivative is 60x3-1500x,whose roots sum to 1500/60=25, for a grand total of 75."

    I really can't understand how it get the sum 50 and 25...Please help me.

    Thanks...
     
  2. jcsd
  3. Nov 25, 2008 #2

    HallsofIvy

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    Science Advisor

    The only thing I can say is that they appear to be talking about the sum of x2 where x is a zero of the polynomial:
    [tex]15x^4-750x^2+735= 15(x^2- 1)(x^2- 49)[/tex]
    so x2= 1 and 49 which add to 50 and
    [tex]60x3-1500x= 60 x(x^2- 25)[/tex]
    so x2= 0 and 25 which add to 25.
     
  4. Nov 25, 2008 #3
    i think so...
    but since we have +7 and -7 and so on...why isn't it 150???
     
  5. Nov 25, 2008 #4
    thank you all the way...
     
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