Solving Trigonometric Limit w/o L'Hospital Rule

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    Limit Trigonometric
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Discussion Overview

The discussion revolves around evaluating the limit of the expression \(\lim_{x\to1}\frac{\sin(x^{3}-1)}{x-1}\) without using L'Hôpital's rule. Participants explore various approaches to solve the limit, focusing on algebraic manipulation and trigonometric identities.

Discussion Character

  • Mathematical reasoning, Homework-related, Exploratory

Main Points Raised

  • One participant expresses uncertainty about how to approach the limit without L'Hôpital's rule.
  • Another participant suggests multiplying the numerator and denominator by \(x^{2} + x + 1\) as a potential method to simplify the limit.
  • A participant acknowledges that multiplying by \(x^{2} + x + 1\) leads to a form that still requires evaluation at \(x \to 1\), indicating they are still unsure how to proceed.
  • There is a mention of rewriting the limit as \(\lim_{x \rightarrow 1} \frac{\sin (x^{3}-1)}{x^{3}-1}(x^{2} + x + 1)\) but questions remain about evaluating both parts of this expression as \(x\) approaches 1.
  • Another participant notes that L'Hôpital's rule could be applied to express the limit in terms of derivatives, specifically stating that it leads to \(3\lim_{x\to1}x^2\cos(x^3-1)\).

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to evaluate the limit without L'Hôpital's rule, and multiple approaches are presented without agreement on a definitive solution.

Contextual Notes

Participants express uncertainty about the steps involved in evaluating the limit, particularly regarding the behavior of the sine function and the polynomial as \(x\) approaches 1. There are unresolved mathematical steps and dependencies on the manipulation of the limit expression.

Who May Find This Useful

This discussion may be of interest to students or individuals seeking alternative methods for evaluating limits in calculus, particularly those looking to avoid L'Hôpital's rule.

Yankel
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Hello,

I need some help with this limit, I have no clue how to do this with the next constraint: No use of L'hopital rule...

Thank you !

\lim_{x\to1}\frac{sin(x^{3}-1)}{x-1}
 
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Yankel said:
Hello,

I need some help with this limit, I have no clue how to do this with the next constraint: No use of L'hopital rule...

Thank you !

\lim_{x\to1}\frac{sin(x^{3}-1)}{x-1}

Try multiplying numerator and denominator by $x^{2} + x + 1$...

Kind regards

$\chi$ $\sigma$
 
chisigma said:
Try multiplying numerator and denominator by $x^{2} + x + 1$...

Kind regards

$\chi$ $\sigma$

I thought about it, it gives me what's in the sine, however x->1 and not x->0, so I am still stuck...
 
Yankel said:
I thought about it, it gives me what's in the sine, however x->1 and not x->0, so I am still stuck...

$\displaystyle \lim_{x \rightarrow 1} \frac{\sin (x^{3}-1)}{x-1} = \lim_{x \rightarrow 1} \frac{\sin (x^{3}-1)}{x^{3}-1}\ (x^{2} + x + 1)$

... and now what is $\displaystyle \lim_{x \rightarrow 1} \frac{\sin (x^{3}-1)}{x^{3}-1}$?...

... and what is $\displaystyle \lim_{x \rightarrow 1} x^{2} + x + 1$?...

Kind regards

$\chi$ $\sigma$
 
Also, L'Hôpital's rule allows us to write:

$\displaystyle \lim_{x\to1}\frac{\sin(x^3-1)}{x-1}=3\lim_{x\to1}x^2\cos(x^3-1)$
 

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