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 .

Re: How to find r for the sum of GP series?

So you are basically just asking how to solve $\displaystyle \frac{1- r^m}{1- r}= s$ for r? You can multiply on both sides by 1- r to get $\displaystyle 1- r^m= s-rs$ which can be reduced $\displaystyle r^m- sr+ (s-1)= 0$. That is an 'm-degree' polynomial. However, there are no formulas, using elementary functions, for solving polynomials of degree greater than 5.

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