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Homework Help: A simple sequences

  1. Oct 12, 2005 #1
    my problem is regarding sequences:

    Sum first 20 terms of a=r^k-1

    terms are 2, 4/3, 8/9,...
    and ratio is 2/3
    and r < 1 so its an inflated series
    and 2(2/3)^19 = .009021859795

    BUT it is an inflated series right? and my answer is the sum of the
    first 20 terms which has to be larger than 2 right? so where am I
    going wrong?
     
  2. jcsd
  3. Oct 12, 2005 #2

    TD

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    Homework Helper

    a=r^(k-1) is a formula for the k-th term, not for the sum of the first k terms.
    The formula for the sum is: [tex]s_n=t_{1}\frac{1-q^n}{1-q}[/tex]
     
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