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Homework Help: Help With Simple Summations

  1. May 31, 2013 #1
    I forgot how to do summations... I've been googling all day, but I can't find resources good enough for the types of problems I have. If anyone can provide me with some resources, I'd really appreciate it.

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

    I'm just going to call that S

    2. Relevant equations
    Ʃ of i = 1 to n is n(n+1)/2

    3. The attempt at a solution
    I know I can separate the two terms:
    S6n + S8

    And then pull the constants out?
    6n*S1 + 8S1

    But can I use 1 as the new constant or do I have to use 4 since that is the index? Do I just multiply 4 by 10087? Or 1 by 10087 and then subtract... something? I don't know, the index of 4 is really throwing me off.
  2. jcsd
  3. May 31, 2013 #2


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    It's not clear what you are trying to sum.

    The summation you have is indexed by 'i', but the summand uses 'n'. Is there some relationship between 'i' and 'n' which you have not posted?

    If you have a formula for a sum which goes from 'i' to 'n', how would you use that formula if 'i' starts at 4?
    Hint: write out the terms of a sum which starts at 1 and goes through say i = 5. How would you modify the sum so that the first term is for i = 4?
  4. May 31, 2013 #3
    I know that there's no relationship between i and n, which is why I'm so lost. If I was using i, and it started at 4, would I just subtract the sum of 1 to 3?
  5. May 31, 2013 #4


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    Of course.
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