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Homework Help: Infinite Series

  1. Oct 29, 2008 #1
    1. The problem statement, all variables and given/known data

    Find the sum of the infinite series: 405-270+180-120+80....

    2. Relevant equations


    3. The attempt at a solution

    I know there's a formula for this but I can't remember it. Could someone refresh my memory?
  2. jcsd
  3. Oct 29, 2008 #2


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    Homework Helper
    Gold Member

    You'll need to start by finding the pattern between successive terms in the series... call the first term [itex]a_0=405[/itex] , the 2nd term [itex]a_1=-270[/itex] etc... so that the series can be represented as:

    [itex]405-270+180-120+80 \ldots=a_0+a_1+a_2+a_3+a_4 \ldots=\sum_{n=0}^{\infty} a_n[/itex]

    There should be any easy to spot relationship between terms in the series.....can you spot it?
    Last edited: Oct 29, 2008
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