## Requiem 2 years ago Power series problem. (x-2)^n/(n^n) Find the radius and interval of convergence.

1. aykayyy

i actually just learned this today lol..... try the ratio test!

2. Requiem

I did, but am not sure if I am doing ti correctly

3. sarahusher

I would do the nth root test?

4. aykayyy

$\lim_{n \rightarrow \infty} \left| \frac{ a _{n+1} }{ a _{n} } \right|$

5. Requiem

|dw:1375316023379:dw|

6. aykayyy

oh yes @sarahusher is probably right! i didnt see the ^n.... the root test would work better

7. Requiem

ok ill try that

8. Requiem

ok i did the root test and got (x-2)/n

9. aykayyy

then take the limit of that

10. sarahusher

Yep!

11. Requiem

i pull the x-2 out, and the limit of (1/n) would be 0

12. sarahusher

exactly!

13. Requiem

but then i multiply the 0 by the (x-2) right?

14. Requiem

which would give me 0 overall?

15. aykayyy

yup

16. sarahusher

As limit < 1, the series will converge for every 'x'

17. Requiem

Im not sure how they are getting the interval of convergence which they are saying is from negative infinity to positive infinity

18. sarahusher

Okay, I'll explain: So when finding the radius of convergence: We know that as the limit=0<1, the series is convergent for every 'x' So for any x for any 'x' you get (ie from -infinity to +infinity) the series will converge Using the limit that you have, you get to ROC = infinity which directly gives you the interval as -infinity<x<infinity

19. sarahusher

Does that make sense?

20. Requiem

Everything else makes sense, but the ROC (x-2) Lim as 'n'-------> infinity of (1/n) = 0 0*(x-2)= 0 Im just not sure how the ROC is infiinty

21. sarahusher

'The radius of convergence' r is a 'nonnegative real number' or '∞' such that the series converges if [x+L] < r , here L is your limit So our limit is 0 so we want an 'r' such that all of our 'x'<r but 'x' always converges for x<r And we worked out before that x converges for all values (ie for all R - real numbers) So it's not so much a calculation to work this out, as opposed to seeing that no matter what your value the series will converge no matter where it lies along R

22. sarahusher

|dw:1375317501046:dw| Think of it like a disc, no matter where your value of 'X' lies in the disc, for this example the series will always converge

23. Requiem

ok that makes more sense now...thanks sara :)

24. sarahusher

Okay, If anything isn't clear let me know :)