# Sequences and series help .

1. Jul 23, 2012

### lionel messi.

Sequences and series help.....

1. The problem statement, all variables and given/known data
3+3a+3a^2+...........∞ is = to 45/8 where a>0,then a is...?

3. The attempt at a solution
since it is a g.p so using
S=(a(rn-1))/(r-1) for r>1
ive all the values except for "n"..can someone help...:/

2. Jul 23, 2012

### eumyang

Re: Sequences and series help.....

Wrong formula. This is an infinite geometric series, so use
$$a + ar + ar^2 + ar^3 + ... = \frac{a}{1 - r}$$

3. Jul 23, 2012

### lionel messi.

Re: Sequences and series help.....

thanks..

4. Jul 23, 2012

### Villyer

Re: Sequences and series help.....

n is the number of terms, so in this case there are infinite terms.
As n tends to infinity, what does r^n tend to? (Assuming -1<r<1)
This gets you to the formula that eumyang posted.