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Math Help - How to find r for the sum of GP series?

  1. #1
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    How to find r for the sum of GP series?

    Hi

    Given such an equation, how do I find r? Thank you

    (1-r^m)/ (1-r)= s
    where m can be any positive whole number
    s can also be any positive number

    The equation is actually the summation for a GP series with a=1, so basically I want to know how to find r (ratio factor) for the sum of a GP .
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  2. #2
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    Re: How to find r for the sum of GP series?

    So you are basically just asking how to solve \frac{1- r^m}{1- r}= s for r? You can multiply on both sides by 1- r to get 1- r^m= s-rs which can be reduced r^m- sr+ (s-1)= 0. That is an 'm-degree' polynomial. However, there are no formulas, using elementary functions, for solving polynomials of degree greater than 5.
    Last edited by HallsofIvy; April 15th 2013 at 01:14 PM.
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  3. #3
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    Re: How to find r for the sum of GP series?

    Hi

    Thanks for the reply.

    Then how do u solve if m greater than 5? Is there an Excel function to do it?

    Thanks
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