Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find sum of roots of polynomial

  1. May 1, 2012 #1
    How would I go about approaching this problem?

    Given the polynomial:
    x^100 - 3x + 2 = 0

    Find the sum 1 + x + x^2 + ... + x^99 for each possible value of x.
     
  2. jcsd
  3. May 1, 2012 #2

    If you meant that x is a root of the polynomial [itex]X^{100}-3X+2[/itex] , then
    [tex]1+x+...+x^{99}=\frac{x^{100}-1}{x-1}=\frac{3x-3}{x-1}=3[/tex]

    DonAntonio
     
  4. May 1, 2012 #3
    Interesting, thanks, how did you derive that?
     
  5. May 1, 2012 #4


    First equality: sum of a geometric sequence.

    Second equality: [itex]x^{100}-3x+2=0\Longrightarrow x^{100}=3x-2[/itex]

    Third equality: trivial algebra

    DonAntonio
     
  6. May 1, 2012 #5
    Oh, I didn't realize the first part was the sum of a geometric series. Thanks for your help.
     
  7. May 2, 2012 #6
    isn't a trivial solution to the equation equal to 1, then then sum would be greater than 3, This is the solution that makes the geometric sum equation impossible as you are dividing by zero.
     
    Last edited: May 2, 2012
  8. May 2, 2012 #7


    Indeed. So for [itex]\,\,x=1\,\,,\,\,1+1^1+1^2+...+1^{99}=100\,\,[/itex] , and for all the other roots it is what I wrote before.

    Thanx.

    DonAntonio
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Find sum of roots of polynomial
  1. Roots of polynomial (Replies: 3)

Loading...