Using L'Hospital's rule to evaluate a fraction containing two variables

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Discussion Overview

The discussion revolves around the application of L'Hospital's rule to evaluate the limit of a fraction involving two variables as they approach zero. Participants explore whether L'Hospital's rule is applicable in this multi-variable context and the implications of the limit's existence.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the meaningfulness of using L'Hospital's rule for the limit \(\lim_{\vec{\mathbf{x}} \rightarrow \vec{\mathbf{0}}}\frac{x^3}{x^2+y^2}\).
  • Another participant hints at the necessity of confirming the limit's existence before applying L'Hospital's rule.
  • Some participants assert that the limit does exist and suggest that it should yield the same value regardless of the path taken to approach zero.
  • There is a discussion on whether L'Hospital's rule can be applied to multi-variable functions, with one participant stating that it can be used if the directional derivative is considered.
  • One participant argues that in the specific example provided, L'Hospital's rule is unnecessary.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of L'Hospital's rule in the context of multi-variable limits. While some believe it can be applied, others suggest it may not be necessary or appropriate.

Contextual Notes

There is uncertainty regarding the conditions under which L'Hospital's rule can be applied to functions of multiple variables, particularly concerning the use of directional derivatives and the existence of limits along different paths.

seeker101
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This might be a silly question:

Is there a meaningful way to use L'Hospital's rule to determine
[tex]\lim_{\vec{\mathbf{x}} \rightarrow \vec{\mathbf{0}}}\frac{x^3}{x^2+y^2}?[/tex]

where
[tex]\vec{\mathbf{x}}=\left(\begin{array}{c}x\\y\end{array}\right)[/tex]
 
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HINT: Are you sure that the limit exists?
 
Last edited:
I think the limit does exist (I am unable to see any reasons for its non-existence; isn't zero the limit?).

But my question is really about using L'Hospital's rule to evaluate
[tex] \lim_{\vec{\mathbf{x}} \rightarrow \vec{\mathbf{0}}}\frac{f(x,y)}{g(x,y)}[/tex]

assuming the numerator and denominator vanish as
[tex] \left(\begin{array}{c}x\\y\end{ar ray}\right)\rightarrow\left(\begin{array}{c}0\\0\end{ar ray}\right)[/tex]
 
seeker101 said:
I think the limit does exist (I am unable to see any reasons for its non-existence; isn't zero the limit?).

But my question is really about using L'Hospital's rule to evaluate
[tex] \lim_{\vec{\mathbf{x}} \rightarrow \vec{\mathbf{0}}}\frac{f(x,y)}{g(x,y)}[/tex]

assuming the numerator and denominator vanish as
[tex] \left(\begin{array}{c}x\\y\end{ar ray}\right)\rightarrow\left(\begin{array}{c}0\\0\end{ar ray}\right)[/tex]
The limit does indeed exists. However, I was hinting that if the limit does exist, then it must have the same value irrespective of the path you take :wink:
 
Thank you for responding. But finding the limit is not my objective. I was wondering if L'Hospital's rule is even applicable in a situation as above (two variable case).
 
seeker101 said:
Thank you for responding. But finding the limit is not my objective. I was wondering if L'Hospital's rule is even applicable in a situation as above (two variable case).
Yes, one may use L'Hopital's rule for multi-variable functions. However, one must use the directional derivative as opposed to the standard derivative. The directional derivative is taken in the direction of the limit.

However, in the example which you have quoted, there is no need to make use of L'Hopital's rule.
 

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