Uncovering the Mystery of an Unlikely Limit Result

  • Context: Undergrad 
  • Thread starter Thread starter Gib Z
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    Limit Mystery
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Discussion Overview

The discussion revolves around the limit \(\lim_{x\to\infty} \sqrt{x^2+x} - x =\frac{1}{2}\), exploring the reasoning behind this result and the methods used to evaluate it. Participants examine different approaches to limits, including algebraic manipulation and approximations, while questioning the validity of their logic and the conditions under which certain methods apply.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion about why their usual logic, which suggests that \(\sqrt{x^2+x}\) should approximate to \(x\) as \(x\) becomes large, does not yield the correct limit.
  • One participant suggests dividing by \(x\) after rationalizing the expression to evaluate the limit correctly.
  • Another participant argues that the logic of replacing polynomials with their leading terms fails in certain cases, as demonstrated by a different limit example.
  • Some participants propose completing the square as a method to simplify the expression and arrive at the limit.
  • There are discussions about the use of first-order approximations and the binomial theorem to understand the behavior of the limit as \(x\) approaches infinity.
  • One participant highlights the importance of ensuring that the degrees of the numerator and denominator are the same for certain methods to work.
  • Several participants discuss the asymptotic behavior of the approximations and the errors involved in different approaches.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method to evaluate the limit or the reasoning behind their approaches. There are multiple competing views on the validity of different techniques and the conditions under which they apply.

Contextual Notes

Some participants mention the limitations of their approximations and the need for higher-order terms to accurately evaluate the limit, indicating that the discussion involves nuanced mathematical reasoning.

Who May Find This Useful

This discussion may be of interest to students and educators in calculus, particularly those exploring limits, approximations, and the subtleties of polynomial behavior at infinity.

  • #31
Gib Z said:
I don't really understand what I'm trying to prove..I know f is continuous, therefore all values are finite, as also shown by your theorem. Since f(x) is finite, and so is f(0), the error must also be finite. So to show that it is less than C|x| for some x we just have to choose a really large C, is that somewhat correct even if not rigorous?

You have to use the fact that f(x) is differentiable as well. Take the function f(x)=x^{1/3}. All values of this function are finite, but the function is not less than C|x| for any C whatsoever when x is sufficiently close to 0.
 

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