Valid method of evaluating limit?

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Homework Help Overview

The discussion revolves around evaluating the limit \(\lim_{x\rightarrow \infty} \frac{x}{\sqrt{a^2+x^2}}\), with participants exploring different methods and reasoning related to limits in calculus.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to use L'Hopital's rule and a substitution method involving trigonometric functions to evaluate the limit. Other participants suggest alternative approaches, including algebraic manipulation by dividing the numerator and denominator by \(x\).

Discussion Status

The discussion is active, with participants providing feedback on the original poster's method and suggesting other valid approaches. There is acknowledgment of different perspectives on how to tackle the problem.

Contextual Notes

Some participants question the necessity of the original poster's substitution method, indicating a preference for more straightforward algebraic techniques. There is also mention of the assumption that \(a^2\) is negligible compared to \(x^2\) as \(x\) approaches infinity.

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Homework Statement


I don't know if the following is valid, so I'll appreciate if someone could tell me if it's ok. I want to find
[tex]\lim_{x\rightarrow \infty} \frac{x}{\sqrt{a^2+x^2}}[/tex]



Homework Equations





The Attempt at a Solution


Using L'Hopital rule doesn't appear to help, because repeatedly differentiating both the top and bottom gives the same limit. So I did a substitution:

[tex]x=a \tan \theta[/tex]
And the problem now becomes [tex]\lim_{\theta \rightarrow \frac{\pi}{2}} \sin \theta[/tex] which easily evaluates to 1. Is this correct?
 
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Physical method would be: a^2 is small compared to infinity, so we can ignore it. Sqrt(x^2)=x, so limit is 1 :P
But yes, I guess your method is just fine.
 
It's valid, but why would you want to do it that way? Just divide numerator and denominator by x and use algebra. Which is what Zizy is saying.
 
OK thanks. I got confused for a moment over something.
 

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