Valid L'Hopitals method for this limit?

  • Thread starter Thread starter Silversonic
  • Start date Start date
  • Tags Tags
    Limit Method
Click For Summary

Homework Help Overview

The problem involves finding the limit of the expression \(\frac{x^{2}\cos(1/x)}{\sin(x)}\) as \(x\) approaches 0. The context suggests a focus on applying L'Hôpital's rule, although it is not explicitly required in the problem statement.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to apply L'Hôpital's rule and questions whether the limit can be expressed as the product of two separate limits. Some participants confirm this property of limits, while others suggest alternative approaches to the factors involved in the limit.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the limit and the application of L'Hôpital's rule. There is no explicit consensus, but guidance has been offered regarding the properties of limits and potential methods for evaluating the expression.

Contextual Notes

Participants are considering the conditions under which limits exist and the nature of the factors in the limit expression, particularly regarding the indeterminate forms present.

Silversonic
Messages
121
Reaction score
1

Homework Statement



Find the limit of

[itex]\frac{x^{2}cos(1/x)}{sin(x)}[/itex] as x tends to 0.

It doesn't actually say with l'hospital's rule, but up until now that's what we've been dealing with.


The Attempt at a Solution



Wolfram alpha tells me the limit is zero. I know already how to compute the limit of [itex]x^{2}cos(1/x)[/itex] as x tends to 0, and this gives zero.

By l'hospital's rule;

lim(x->0) [itex]\frac{f(x)}{g(x)}[/itex] = lim(x->0) [itex]\frac{f'(x)}{g'(x)}[/itex]


Can I represent lim(x->0) [itex]\frac{f(x)}{g(x)}[/itex] as lim(x->0) f(x) * lim(x->0) [itex]\frac{1}{g(x)}[/itex]?

And thus if I show f(x) tends to a value, and 1/g(x) tends to a value, then the whole thing tends to a value? Taking f(x) = [itex]x^{2}cos(1/x)[/itex] and 1/g(x) as [itex]\frac{1}{sin(x)}[/itex], then I can compute the limit of [itex]\frac{1}{sin(x)}[/itex] as the limit of [itex]\frac{e^{x}}{e^{x}sin(x)}[/itex]. Using l'hospital's rule once on this gives me a limit of one. Thus the limit of the whole thing I've wanted is 0/1, so 0.

Is this correct?
 
Physics news on Phys.org
Silversonic said:
Can I represent lim(x->0) [itex]\frac{f(x)}{g(x)}[/itex] as lim(x->0) f(x) * lim(x->0) [itex]\frac{1}{g(x)}[/itex]?

Sure you can, it's a property of limits:
eq0005M.gif
 
As long as the limits exist, of course.
 
I would think of it as (x/sin(x))*(x*cos(1/x)). I'd use l'Hopital for the first factor, if you don't already know it. The second factor isn't all that indeterminant, is it? Use another test on the second factor.
 

Similar threads

  • · Replies 12 ·
Replies
12
Views
1K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 13 ·
Replies
13
Views
4K
Replies
3
Views
1K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 11 ·
Replies
11
Views
2K