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Math Help - Summing a series problem

  1. #1
    iva
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    Summing a series problem

    Hi There, i am trying to sum up a sequence of numbers using the
    summing rule for
    1 + a^1 + a^2 + a^3 + a^4 +... + a^n = (a^n - 1/(a-1),

    for this sequence:

    1/9 + 1/9^2 + 1/9^3 + .. + 1/9^n.

    Is it as simple as just inverting the formula ie :
    ((a^n - 1/(a-1))^-1 ?? What about the 1 that doesn't exist at the
    beginning, normally i would subract a 1 after my calculation but if I
    do that here the answer will be negative.

    Any help appreciated
    Many thanks
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  2. #2
    Math Engineering Student
    Krizalid's Avatar
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    Santiago, Chile
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    the sum is \frac{a^{n+1}-1}{a-1} and put a=\frac19.
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