# Nth partial sum of a series Σ Ak is Sn = (n-1)/(n+1)

1. Oct 20, 2004

### Kenshin

If the nth partial sum of a series Σ Ak is Sn = (n-1)/(n+1) , find Ak . Does Σ Ak converge?

i looked in my math book and cant find how to do this.

2. Oct 21, 2004

### Tide

Try subtracting S(n) from S(n+1).

3. Oct 21, 2004

### HallsofIvy

Staff Emeritus
Either I or Tide misunderstands your question. Subtracting Sn from Sn+1 will give the An+1 but won't tell you whether or not the sequence converges.

The DEFINITION of convergence of a series is the convergence of the sequence of partial sums.

Can you find the limit of the sequence $\frac{n-1}{n+1}$ as n goes to infinity??

4. Oct 21, 2004

### Kenshin

Ak= 2/x^2+3x+2. so you subtrack Sn+1-Sn. can u show me how that works. it just seems weird that, that is all u have to do. thanks

5. Oct 23, 2004

### TenaliRaman

Kenshin,
Look at the last statement made by HallsOfIvy ...
Have u worked out what he said?

-- AI

6. Oct 23, 2004

### shmoe

To find $$A_k$$ yes, this is all you have to do (you replaced your "k" with "x" though, and also "n"). Do you understand why this works?

To answer the question of convergence for the series, see HallsofIvy's post. You have to look at $$\lim_{n\rightarrow\infty}S_n$$

7. Oct 23, 2004

### matt grime

Let A(k) be any series, and S(n) the sum of the first n terms, then S(n+1) is the sum of the first n terms plus the n+1'st term. Can you now prove the result that puzzles you?

8. Oct 23, 2004

### HallsofIvy

Staff Emeritus
I DID misunderstand the question! There were TWO things asked:

1) Find A(k) which is what Tide was responding to.

Since S(n), the "partial sum is defined as A(1)+ A(2)+ ...+ A(n),
A(n)= S(n)- S(n-1)= (n-1)/(n+1) - (n-1-1)/(n-1+1)= (n-1)/(n+1)- (n-2)/n
= n(n-1)/n(n+1)- (n+1)(n-2)/n(n+1)= (n2-n-n2+n+2)/n(n+1)
= 2/n(n+1).

2) Since A(k) converges if and only if the sequence of partial sums converges, look at
S(n)= (n-1)/(n+1). Divide both numerator and denominator by n: (1- 1/n)/(1+ 1/n).
As n goes to infinity, 1/n goes to 0 so the S(n) converges to 1.

Yes, &Sigma; A(k) converges. In fact, it converges to 1.

9. Oct 23, 2004

### matt grime

I don't think you mean if and only if in part 2) there.

10. Oct 23, 2004

### HallsofIvy

Staff Emeritus

??? That's certainly the definition of "convergence of a series" that I learned.

If the sequence of partial sums converges, then the series converges to the same limit.

If the sequence of partial sums does not converge, then the series does not converge.

11. Oct 23, 2004

### shmoe

I inserted missing Σ?

12. Oct 24, 2004

### matt grime

The reason I didn't put a correction is that two spring to mind. A(k) converges if the sequence of partial sums converges, or by definition the sum of the sequence converges if and only if the sequence of partial sums converges, and I wasn't sure which Hallsofivy meant. It seems from your reply, that you intended the second option.

13. Oct 24, 2004

### HallsofIvy

Staff Emeritus
Oh- I forgot the &Sigma;!