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Summation problem (first N positive integers)

  1. Oct 5, 2013 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    so i kno the formula for the for the sum of the first N positive integers


    when i = 1

    3. The attempt at a solution

    i kno the answer = n^2(n+1)/2

    but could someone explain step by step how you reduce it to get the final answer? as if i'm in kindergarten? i'm slow, thanks.

    my work:

    following the formula: n(n+1)/2 * n(n+1)/2 * n(n+1)/2 ?

    i don't understand how to reduce the terms to get the final answer provided
    Last edited: Oct 5, 2013
  2. jcsd
  3. Oct 5, 2013 #2

    Ray Vickson

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    PF rules require you to show us your work.
  4. Oct 5, 2013 #3
    oh okay sure
  5. Oct 5, 2013 #4


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    Are you sure the upper bound on the second summation is "i" and not "n"? Note that this is a summation of the value "1", not the variable "i".
  6. Oct 5, 2013 #5


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    If the upper bound on the second summation were "n" then the sum would be n^3. You are only going to get the correct answer if the upper bound is "i". Start with the innermost sum and work your way out. It's not hard.
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