Calculating Limits Involving Cosine: Is My Approach Correct?

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    Cosine Limits
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Homework Help Overview

The discussion revolves around calculating the limit of a function involving the cosine function as x approaches zero. The specific limit in question is \(\lim_{x\rightarrow0}\frac{\sqrt{5-\cos(x)}-2}{x^{2}}\), which falls under the subject area of calculus, particularly limits and trigonometric functions.

Discussion Character

  • Exploratory, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the approach of multiplying by the conjugate of the numerator to simplify the limit expression. There are mentions of dividing by \(x^2\) and alternative methods involving the Taylor series expansion of cosine.

Discussion Status

Some participants express understanding of the approaches taken, while others share insights about different methods for handling the cosine function. There is a general exchange of ideas without a clear consensus on the correctness of the original poster's method.

Contextual Notes

Participants note that certain techniques, such as using the Taylor series expansion for cosine, may not have been covered in their current curriculum, indicating a potential gap in knowledge that could affect their understanding of the problem.

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Homework Statement


find the limit of :

[tex]\lim_{x\rightarrow0}\frac{\sqrt{5-cos(x)}-2}{x^{2}}[/tex]


Homework Equations


The Attempt at a Solution



I multiplied the numerator and the denominator by the conjugate of the numerator and i got :

[tex]\frac{1-cos(x)}{x^{2}(\sqrt{5-cos(x)}+2)}[/tex]

then: i divided top and bottom by x^2 and i got (1/2)/4 which is 1/8.

Is what I have done correct?
 
Last edited:
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hi mtayab1994! :smile:
mtayab1994 said:
I multiplied the numerator and the denominator by the conjugate of the numerator and i got :

[tex]\frac{1-cos(x)}{x^{2}(\sqrt{5-cos(x)}+2)}[/tex]

then: i divided top and bottom by x and i got (1/2)/4 which is 1/8.

Is what I have done correct?

oooh, I've never seen that trick before! :-p

yes, that's fine

(the usual way of dealing with cosx is to replace it by 1 - x2/2 … same result)
 
Yea thanks. And by the way I didn't know that you replace cos x with 1-x^2/2. Well maybe it's because we haven't done it yet.
 
ah!

yes, cos x = 1 - x2/2 + x4/4! - x6/6! + …

you'll learn about that later :smile:
 
tiny-tim said:
ah!

yes, cos x = 1 - x2/2 + x4/4! - x6/6! + …

you'll learn about that later :smile:

Alrighty thank you very much. I live in morocco and I'm in kind of like in a "Math-Science" major in high school so we're really big on math and physics over here.
 

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