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Sum and Sum to infinity

  1. Aug 31, 2009 #1
    1. The problem statement, all variables and given/known data

    Hi there, question asks "What is the difference between the sum to ten terms and the sum to infinity. a = sqroot 2 r = sqroot 2/2

    The sum to ten terms, I worked out as 31 + 31 sqroot 2
    The sum to infinity, I worked out as 2 sqroot 2 + 2

    2. Relevant equations

    Is there one equation to cover this type of problem or do I need to subtract one equation from the other to get one equation and hence one answer?

    3. The attempt at a solution

    Totally confused?
     
  2. jcsd
  3. Aug 31, 2009 #2
    And what are those a and r?

    Is a the first term of the series, and r the common ratio?

    If so, the sum to n numbers of geometric series
    [tex]S_f=a\frac{1-r^n}{1-r}[/tex]

    The sum of infinite numbers of geometric series
    [tex]s \;=\; \sum_{k=0}^\infty ar^k = \frac{a}{1-r}.[/tex]
     
  4. Sep 1, 2009 #3

    HallsofIvy

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    It would have been a good idea to say that this is a geometric series! Given that, njama is correct.
     
  5. Sep 1, 2009 #4
    Many thanks for your replies. You have clarified my thoughts.

    Yes, my apologies I should have specified a G.P.

    Cheers Petra d.
     
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