Find limits for x to infinity with L'Hopital

In summary, the given function is being analyzed to determine if it tends to a limit as x approaches infinity. L'Hopital's rule is not applicable and a more basic approach is needed. Using the squeeze theorem, it can be shown that the function will tend to a limit as x approaches infinity.
  • #1
Fairy111
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Homework Statement



For the following function decide whether f(x) tends to a limit as x tends to infinity. If the limit exists find it.

Homework Equations



f(x)=[xsinx]/[x^2 +1]

The Attempt at a Solution



I thought about using L'Hopitals rule, so i got:

[sinx + xcosx]/[2x]

So, sinx will keep between 0 and 1, and the xcosx and the 2x would both tend to infinity maybe?

Im not really sure though.
 
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  • #2


l'Hopital's rule is going to get you into trouble here. x^2+1 goes to infinity, but x*sin(x) does not. It's oscillates. So l'Hopital doesn't apply. You'll have to approach the limit in a more basic way. How about dividing numerator and denominator by x?
 
  • #3


A different approach is to use the fact that for x>=0, -x <= xsinx <= x, which means that (again for x >= 0)
[tex]\frac{-x}{x^2 + 1} \leq \frac{x sin(x)}{x^2 + 1} \leq \frac{x}{x^2 + 1} [/tex]

The outermost expressions both have limits as x approaches infinity, and you can exploit this fact to say something about the expression in the middle. This idea is referred to in some texts as the "squeeze" theorem or "squeeze-play" theorem.
 

Related to Find limits for x to infinity with L'Hopital

What is L'Hopital's rule?

L'Hopital's rule is a mathematical rule used to find the limit of a function as the independent variable approaches infinity or negative infinity. It is also known as the rule of de L'Hopital or the l'Hospital's rule.

When is L'Hopital's rule used?

L'Hopital's rule is used when the limit of a function in indeterminate form (such as 0/0 or ∞/∞) needs to be evaluated. It helps to simplify the expression by taking the derivative of the numerator and denominator separately and then re-evaluating the limit.

What are the conditions for using L'Hopital's rule?

The conditions for using L'Hopital's rule are that the limit must be in an indeterminate form, the function must be differentiable in a neighborhood of the limit point, and the limit of the derivative of the function exists.

Can L'Hopital's rule be used for any type of limit?

No, L'Hopital's rule can only be used for limits where the independent variable approaches infinity or negative infinity. It cannot be used for limits where the independent variable approaches a finite value or a specific point.

Are there any alternatives to using L'Hopital's rule?

Yes, there are alternative methods for evaluating limits such as direct substitution, factoring, and using trigonometric identities. However, L'Hopital's rule is often the most efficient and accurate method for finding limits involving indeterminate forms.

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