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From: JEMebius on 16 Mar 2010 08:34
Albert wrote:
> The sum of the first four terms of a geometric series is 30 and the
> sum to infinity is 32.
> Find the first two terms.
>
> What have I done wrong?
> 30 = ar^3
> 32 = a / (1 - r)
>
> 30 / r^3 = a
> 32(1 - r) = a
>
> 30 / r^3 - 32 + 32r = 0
> 30 - 32r^3 + 32r^4 = 0
> 15 - 16r^3 + 16r^4 = 0


I guess you read the problem too fast and got hooked on the word "four".
Indeed: the fourth term in the geometric series [a, a.r, a.r^2, a.r^3, ...] is a.r^3.

So you tried to solve the problem
"The fourth term of a geometric series is 30 and the sum to infinity is 32.
Find the first two terms."

But that was not the problem...


Happy studies and happy reading: Johan E. Mebius
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