Finding the Limit of (1-cos(a*x))/x^2: How to Solve

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Discussion Overview

The discussion revolves around finding the limit of the expression (1 - cos(a*x))/x^2 as x approaches 0. Participants explore various methods to approach the limit, including power series, L'Hôpital's rule, and graphical analysis. The scope includes mathematical reasoning and conceptual understanding of limits.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses uncertainty about how to approach the limit problem and notes that their teacher mentioned the answer would involve 'a'.
  • Another participant suggests that the limit approaches positive infinity, but acknowledges the possibility of being incorrect.
  • A different participant challenges the previous claim, proposing the use of the power series expansion for cos(ax) to derive the limit.
  • Another participant states that using L'Hôpital's rule would simplify the problem, providing a step-by-step approach to arrive at a limit of a^2/2.
  • Some participants express concern about not being familiar with L'Hôpital's rule and seek alternative methods to derive the limit.
  • One participant suggests using the Maclaurin series for cos(ax) to approximate the limit.
  • Another participant mentions the possibility of using graphical methods to observe the behavior of the function for various values of 'a'.
  • Some participants discuss the use of delta-epsilon proofs as a rigorous method to establish the limit without relying on series expansions or L'Hôpital's rule.

Areas of Agreement / Disagreement

There is no consensus on the correct method to find the limit, with multiple competing views and approaches presented. Some participants advocate for the use of series expansions, while others suggest graphical analysis or L'Hôpital's rule.

Contextual Notes

Participants express uncertainty about their familiarity with certain mathematical tools, such as L'Hôpital's rule and Maclaurin series, which may limit their ability to engage with the problem fully. The discussion also highlights varying levels of mathematical background among participants.

Who May Find This Useful

This discussion may be useful for students learning about limits, particularly in the context of trigonometric functions, as well as those exploring different methods of mathematical proof and approximation.

Caldus
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I am not sure what the answer to this is or how to go about getting an answer for this:

Find:

lim (1 - cos(a*x))/x^2
x->0

My teacher said the answer would have the term 'a' in it.

How do I go about getting the answer to a limit problem like this? Thank you.
 
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[tex]\lim _{x \rightarrow 0} \frac{1 - \cos ax}{x^2} = \frac{1 - \cos 0}{0^2} = \infty ^+[/tex]
I might be wrong though, but I think the limit is (positive) infinity and that the function has an asymptote at [tex]x = 0[/tex].
 
Last edited:
Sorry to say that Chen, but you are wrong. To solve this, look at the power series of cos(ax). Like this you will have [tex]1/2*a^2x^2+O(x^4)[/tex] on top of the line and another square term below. So the limit is...

You can also use L'Hôpital (twice), if you know it.
 
Last edited:
It's trivial with l'Hopital's rule, but your teacher may or may not want you to use that ;)

lim (x -> 0) (1 - cos(a*x)) / x^2 =
lim (x -> 0) a * sin(ax) / (2x) =
lim (x -> 0) aa * cos(ax) / 2 =
a^2 * 1/2 =
a^2/2
 
I figured it was too simple to be true. :wink:
 
I'm not supposed to know about L'Hopital's rule yet. How could I come up with that formula? I have to show some kind of evidence. I don't know how to come up with that formula. Thanks.
 
Hmm, [tex]\lim _{x \rightarrow 0} \frac{1 - \cos ax}{x^2}[/tex]

Using the maclaurin series:

[tex]\cos ax \approx 1 - \frac{(ax)^2}{2}[/tex]

Try that
 
I do not think I am supposed to know that either. Is there an easy way to prove that the limit is (ax^2)/2?
 
You could always graph it on a calculator and point.

cookiemonster
 
  • #10
Yeah I know I did that for when a = 1, 1.5, and 2. But I don't know how to find the formula for finding the limit if a was anything.
 
  • #11
Anyone know? Thanks.
 
  • #12
Er, I guess you could always try delta-epsilon proofs, but I don't think you want to go there.

Why not just plot it for a bunch of values of a and look for a pattern? If you have 20 values of a of varying value supporting you, it's pretty solid for your purposes.

cookiemonster
 
  • #13
Suppose you are allowed to know the limits: lim(x->0)(sin(x)/x)=1,lim(x->0)(cos(x)=1),
im(x->0)(a(x)*b(x))=lim(x->0)a(x))*(lim(x->0)b(x),
lim(x->0)(a(x)/b(x))=(lim(x->0)a(x))/(lim(x->0)b(x))
Then you may do without MacLaurin series/L'Hopital's Rule in your proof
 
  • #14
cookiemonster said:
Er, I guess you could always try delta-epsilon proofs, but I don't think you want to go there.

Why not just plot it for a bunch of values of a and look for a pattern? If you have 20 values of a of varying value supporting you, it's pretty solid for your purposes.

cookiemonster

now that's very rigorous
 
  • #15
Caldus said:
I do not think I am supposed to know that either. Is there an easy way to prove that the limit is (ax^2)/2?
Then would you mind explaining what areas you have covered around this sort of area?
 

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