Can l'Hôpital's Rule Help Determine the Limit of an Improper Integral?

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Discussion Overview

The discussion centers around the application of l'Hôpital's Rule in determining the limit of an expression involving logarithmic functions as it approaches an indeterminate form. Participants explore whether l'Hôpital's Rule is suitable for this scenario and suggest alternative methods, including induction and substitution.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant requests help in proving that \(\lim_{x\rightarrow 0}x({\ln x})^n=0\) for all positive integers \(n\).
  • Another participant suggests using l'Hôpital's Rule combined with induction to tackle the limit problem.
  • A different participant expresses frustration with using l'Hôpital's Rule, noting that it consistently results in an indeterminate form of infinity/infinity.
  • One participant proposes a substitution \(x=e^{-v}\) as a more effective approach, arguing that it simplifies the comparison between exponential and polynomial functions.
  • Another participant supports the use of l'Hôpital's Rule and induction, claiming it works well for their understanding of the problem.
  • A later reply humorously questions the understanding of the comparison between exponential and polynomial functions if one is not familiar with it, suggesting that l'Hôpital's Rule is a necessary tool.

Areas of Agreement / Disagreement

Participants express differing opinions on the effectiveness of l'Hôpital's Rule for this limit problem. Some advocate for its use, while others argue that it is not suitable and recommend alternative methods. The discussion remains unresolved regarding the best approach.

Contextual Notes

Participants highlight the challenges of applying l'Hôpital's Rule, particularly in deriving reasonable expressions for derivatives involving logarithmic functions. There is also mention of the need for familiarity with the comparison between exponential and polynomial functions, which may affect the understanding of the problem.

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Proving an indeterminate form

Prove for all positive integers n that [tex]\lim_{x\rightarrow 0}x({lnx})^n=0[/tex]

Thanks for any help.
 
Last edited:
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That's not an integral.

Do you know l'Hopital's rule? Combine it with induction.
 
oh...i didn't think of induction
i kept doing it by l'hopital's rule and it came out infinity/infinity all the time.
Thanks for the advice
 
It's futile to use L'Hôpital's rule (you can't get a reasonable expression for

[tex]\frac{d^{k}(\ln x)^{n}}{dx^{k}}[/tex]

)

Do a substitution:

[tex]x=e^{-v}[/tex]

The result is immediate.It's like comparing exp & a finite polynomial.Since "n" is fixed,the factor [itex](-1)^{n}[/itex] bears no relevance...

Daniel.
 
L'Hôpital's rule + induction works fine for me... just like Jameson said.
 
dextercioby said:
It's like comparing exp & a finite polynomial.

and how do you know what happens in this comparison if you aren't familiar with it? enter l'Hôpital... :-p
 

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