# Sum and Sum to infinity

1. Aug 31, 2009

### zebra1707

1. The problem statement, all variables and given/known data

Hi there, question asks "What is the difference between the sum to ten terms and the sum to infinity. a = sqroot 2 r = sqroot 2/2

The sum to ten terms, I worked out as 31 + 31 sqroot 2
The sum to infinity, I worked out as 2 sqroot 2 + 2

2. Relevant equations

Is there one equation to cover this type of problem or do I need to subtract one equation from the other to get one equation and hence one answer?

3. The attempt at a solution

Totally confused?

2. Aug 31, 2009

### njama

And what are those a and r?

Is a the first term of the series, and r the common ratio?

If so, the sum to n numbers of geometric series
$$S_f=a\frac{1-r^n}{1-r}$$

The sum of infinite numbers of geometric series
$$s \;=\; \sum_{k=0}^\infty ar^k = \frac{a}{1-r}.$$

3. Sep 1, 2009

### HallsofIvy

Staff Emeritus
It would have been a good idea to say that this is a geometric series! Given that, njama is correct.

4. Sep 1, 2009

### zebra1707

Many thanks for your replies. You have clarified my thoughts.

Yes, my apologies I should have specified a G.P.

Cheers Petra d.