# Homework Help: Not sure how to proceed with this series

1. Dec 10, 2007

### frasifrasi

With this series,

Summation from 2 to infinity of

(-1)^(n).n/ln(n)

--> Am I supposed to do the alternating series test?

If I do, lim n --> infinity n/ln(n): am I supposed to do l'hopital here or is there a more straight foward method?

Thank you.

2. Dec 10, 2007

### Office_Shredder

Staff Emeritus
L'hopital is a pretty good way to do $$\frac {n}{ln(n)}$$ as n goes to infinity

3. Dec 10, 2007

### frasifrasi

ok, it goes to infinity, but the first part of the alternating series, how can I determined that $$\frac {n+1}{ln(n+1)}$$?

also, for a different series altogether, I am not sure how $$\frac {(-1 to (n-1))}{2n}$$ --> -1^(n-1)
is conditionally convergent. If I take the abs value, I get 1/2n, which is a p series and diverges. And the alternating series diverges, so would it not diverge?

Last edited: Dec 10, 2007
4. Dec 10, 2007

### Office_Shredder

Staff Emeritus
To be completely honest, I have no idea what you're trying to ask. Did you mess up the latex?

The fact that the limit goes to infinity specifically means it doesn't go to zero, which is a necessary condition for convergence

5. Dec 10, 2007

### frasifrasi

but the first series I posted is supposed to be conditionally convergent. Can anyone else help?

6. Dec 10, 2007

### Avodyne

It isn't.

7. Dec 11, 2007

### frasifrasi

That is what it says in the answer key, condinionally...

Can anyone else comment?

8. Dec 11, 2007

### Dick

Divergent. The key is simply wrong or the problem is misstated.

9. Dec 11, 2007

### HallsofIvy

Are you talking about the same series?

The "last series" referred to, I think, was
$$\sum_{n=1}^\infty \frac{(-1)^{n-1}}{n}$$
an "alternating series". That converges if and only if the associated sequence converges to 0 which is true. Of course, the series of absolute values is
$$\sum_{n=1}^\infty \frac{1}{n}$$
which does not converge so the sum converges "conditionally", not "absolutely".

For the first series
$$\sum_{n= 1}^\infty \frac{n}{ln n}[/itex] Since the associated sequence n/ln n does not go to 0 it cannot converge either conditionally nor absolutely. 10. Dec 12, 2007 ### frasifrasi so, [tex]\sum_{n= 1}^\infty \frac{n}{ln n}$$ --> the lim n-->infinity =

1/ 1/x = infinity, so it diverges, correct?

11. Dec 12, 2007

### Office_Shredder

Staff Emeritus
This would be better stated as
1/(1/n) -> infinity as n-> infinity, so n/ln(n) -> infinity as n->infinity by L'Hopital. Hence, the sum is divergent, as the summation terms do not tend to zero

12. Dec 12, 2007

### frasifrasi

What about $$\sum_{n= 1}^\infty \frac{6^n + 5^n}{30^n}$$

- it's been a while, what method should I use for this?

13. Dec 12, 2007

### frasifrasi

anyone (I am trying to prepare for a test : ) !)

14. Dec 12, 2007

### Avodyne

Can you do $\sum_{n= 1}^\infty x^n$ for $|x|<1$?

15. Dec 12, 2007

### VietDao29

Big hint of the day: You can split it into 2 small parts, and solve it easily using the sum of geometric series.

Can you go from here? :)