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Geometric sequence question in IB HL mathematics paper 1 november 2010

  1. Apr 13, 2013 #1
    1. The problem statement, all variables and given/known data

    The sum S n of the first n terms of a geometric sequence whose nth term is u n is given by

    7n-an / 7n

    Where a > 0

    Find an expression for un

    Find the first term and the common ratio of the sequence

    Consider the sum to infinity of the sequence

    Determine the values of a such that the sum to infinity exists
    Find the sum to infinity when it exists

    I tried to make it equal to the equation for Sn but it doesn't seem to be helping.

    Thanks in advance
     
  2. jcsd
  3. Apr 13, 2013 #2

    Ray Vickson

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    Do you mean
    [tex] u_n = \frac{7^n - a^n}{7^n},[/tex]
    of do you mean
    [tex] u_n = 7^n - \frac{a^n}{7^n}?[/tex]
    Use parentheses, like this: (7^n - a^n)/7^n or 7^n - (a^n/7^n).
     
  4. Apr 13, 2013 #3

    haruspex

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    That's the right way. Pls post your working.
     
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