Valid L'Hopitals method for this limit?

In summary, the conversation discusses finding the limit of a function using L'Hopital's rule and other methods. It is determined that the limit is zero, with the use of Wolfram Alpha and the application of various techniques. The idea of representing the limit as the product of two individual limits is also discussed.
  • #1
Silversonic
130
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'Hopitals 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'Hopitals 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'hopitals 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?
 
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  • #2
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:
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  • #3
As long as the limits exist, of course.
 
  • #4
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.
 

FAQ: Valid L'Hopitals method for this limit?

What is L'Hopital's method?

L'Hopital's method is a mathematical technique used to evaluate limits of functions that cannot be solved using traditional methods. It was developed by French mathematician Guillaume de l'Hopital in the 17th century.

When is L'Hopital's method applicable?

L'Hopital's method is applicable when the limit of a function can be written in the form of 0/0 or infinity/infinity. In other words, when the numerator and denominator both approach 0 or infinity as x approaches a certain value.

3. What is the process for using L'Hopital's method?

The process for using L'Hopital's method is as follows:

  1. Identify the limit of the function in question.
  2. If the limit is in indeterminate form (0/0 or infinity/infinity), take the derivative of the numerator and denominator separately.
  3. Simplify the resulting derivative expression.
  4. Take the limit of the simplified expression.
  5. If the new limit is still in indeterminate form, repeat the process until a definite value is obtained.

4. In what situations should L'Hopital's method not be used?

L'Hopital's method should not be used when the limit of a function can be solved using traditional methods, such as algebraic manipulation or substitution. It should also not be used when the limit is not in indeterminate form.

5. Can L'Hopital's method be used to solve limits at infinity?

Yes, L'Hopital's method can be used to solve limits at infinity, as long as the limit is in indeterminate form (infinity/infinity). In this case, the process is the same as for limits at a specific value, except that the limit is taken as x approaches infinity.

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