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Expert: Sherry Wallin - 12/16/2011


Question
From the formula for the sum of a geometric progression(GP). S= a(r^n - 1)/r - 1 make "r" the subject of the formula.  ? can only solve this assuming "n" was replaced by 2 or 3, aside from that I've tried to ?  avail. Please is the solution feasible. Tanks

Answer
I'm not 100% certain what you are trying to do. If what you are trying to do is solve for r it cannot be done because:
S = a(r^n-1)/(r-1) =>
S/a = (r^n-1)/(r-1) =>
S/a = (r-1)(r^n-1 + r^n-2 + ... + r^2 + r + 1)/(r-1) =>
S/a =(r^n-1 + r^n-2 + ... + r^2 + r + 1)
and as you can see you cannot get r by itself...

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Sherry Wallin

Expertise

I can answer most questions up through Calculus and some in Number Theory and Abstract Algebra.

Experience

I have had my Bachelor's Degree since 1987 and have been a teacher since 1988. I earned my Masters Degree in Mathematics May 2010. I have been teaching at the same community college since 2002.

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I have taught 12 years at the community college level, medical college, and technical college as well as a high school instructor and alternative education instructor and charter school instructor.

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Master's GPA 3.56 Bachelor's GPA 3.34 Post grad work not degree related GPA 4.0

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