Limits splitting the fraction into two

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Discussion Overview

The discussion revolves around evaluating the limit of the expression (x2n-1)/(x2n+1) as n approaches infinity. Participants explore various approaches to solve the limit, considering different cases based on the value of x, and express challenges in reaching a definitive conclusion.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests splitting the fraction and applying different methods, but finds them unhelpful.
  • Another participant proposes considering various cases for x (e.g., x > 1, x = 1, x < -1) to evaluate the limit.
  • For x > 1, one participant claims the limit approaches 1, while for x = 1, the limit is 0, and for 0 < x < 1, the limit is -1.
  • Questions arise regarding the limit for x < 0, with uncertainty expressed about the nature of negative powers and whether the limit does not exist in that case.
  • A participant notes that the limit's behavior depends on the size of x, indicating that if x is small, 1/x becomes large, and vice versa.
  • There is an acknowledgment that n is typically considered an integer, which may influence the interpretation of the limit.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the limit's value across different cases of x, and multiple competing views remain regarding the behavior of the limit as n approaches infinity.

Contextual Notes

Participants express uncertainty about the implications of fractional powers of negative numbers and the assumptions regarding the nature of n as an integer.

StephenPrivitera
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limn-->oo(x2n-1)/(x2n+1)
I can't figure this one out. I've tried everything. I tried splitting the fraction into two, applying a log to each side, factoring the top, dividing by x2n, and Lhopitals rule doesn't apply and wouldn't help if it did. Any ideas?
 
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perhaps the only way to do this would be to consider various cases such as x>1, x=1, x<-1, x=-1, etc.
anyone agree?

I get for x>1
lim=1
for x=1, lim=0
if 0<x<1, lim=-1

What about x<0? What is -2^999999.5? Surely, it's not real. Can we say that lim DNE for x<0?
For -1<x<0 x^n would be very small, but wouldn't n have to be some integer?
 
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I got stuck.

For example,
I'll omit the subscripts.

lim(x2n-1)/(x2n+1)=lim(1-1/x2n)/(1+1/x2n)
As n grows to infinity, we can say nothing about the limit, because it depends on what x is. If x is small then 1/x is large. If x is big, then 1/x is small.


edit: whoops edited wrong post, sorry.
 
Last edited by a moderator:
Grargh, you read and responded before I could delete my post.

Yes, breaking it up into cases is a good idea.

edit: n often implicitly means an integer, and it wouldn't surprise me if this problem assumed as such.

*sigh* Today isn't my best day. :wink:
 
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I would interpret this as n being an integer so you don't have any problems with fractional powers of negative numbers.

Because x2n= (x2)n) it doesn't matter whether x is positive or negative so you might as well assume positive. In that case the crucial cases are: 0<= x<1, x= 1, x> 1.
 

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