1. ## Geometric sequences

im really stuck on this question.

The sum of the first 4 terms ofa geometric series is 30, and the sum of the infinite series is 32. Find the first 3 terms.

i know i have to find a and r, however Im not sure how to do it.
Do i work out what a is for both sides??? HELP!!!.. PLEASE

2. ## Re: Geometric sequences

The sum of a series for the first n terms is $a\frac{1-r^n}{1-r}$

The sum of an infinite geometric series (assuming |r|<1) is $\frac{a}{1-r}$

$a\frac{1-r^4}{1-r}=30$
$\frac{a}{1-r}=32$
$a\frac{1-r^4}{1-r}= \frac{a}{1-r}\cross (1-r^4)=30$