Intersections of a line and a curve

  1. 1. The problem statement, all variables and given/known data

    For all all lines which meet the graph y = 2x^4+7x^3+3x-5 at four distinct points, what is the sum of the x-coordinates of the four points of intersection?


    2. Relevant equations



    3. The attempt at a solution

    So, you obviously set ax+b =2x^4+7x^3+3x-5, but how do you find the zero's of that?
     
    Last edited: Jul 21, 2007
  2. jcsd
  3. Hurkyl

    Hurkyl 16,090
    Staff Emeritus
    Science Advisor
    Gold Member

    You don't need to know the roots; you only need to know their sum.
     
  4. Yes and how do you know their sum without knowing the roots?
     
  5. Gib Z

    Gib Z 3,348
    Homework Helper

    http://en.wikipedia.org/wiki/Viète's_formulas

    Its a bit of grinding but in this case its a degree 4 polynomial so we can get it to the form [itex](x-\alpha)(x-\beta)(x-\gamma)(x-\delta)[/itex]. Where the greek letters are the roots. Now expand that, and equate to co efficients and we can get an expression for the sum of the roots knowing only the coefficients.
     
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