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Homework Help: Sum of series

  1. Jul 27, 2008 #1
    1. The problem statement, all variables and given/known data

    [tex]\Sigma[/tex] (1/2^n+1/3^n)
    2. Relevant equations

    3. The attempt at a solution
    I get 7/2
    by using a/1-r to both than adding....what am I doing wrong

    2+3/2= 7/2
  2. jcsd
  3. Jul 27, 2008 #2


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    Homework Helper

    I didn't get 7/2. Your technique seems ok. Must be some calculation error.
  4. Jul 27, 2008 #3


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    Homework Helper

    You are using a/(1-r) with a=1. a is supposed to be the first term in the series. The sum starts at n=1.
  5. Jul 27, 2008 #4


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    Science Advisor

    The standard geometric series starts at n=0. These are not geometric series because they are missing the "n=0" term. Of course, for each of the two series, that would just be 1/20= 1 and 1/30= 1. Since those terms are missing, you need to subtract 2 from your result.
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