Cosine IND limit

  • Thread starter Hernaner28
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  • #1
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It's the following one:

[tex]\displaystyle\lim_{x \to{0}}{\frac{1-\cos(1-\cos x)}{3x^4}}[/tex]

In case we have to apply L'Hospital, appart from it, how could I solve this without it?
Thanks!
 

Answers and Replies

  • #2
Dick
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You could use the Taylor series expansion of cos(x) around x=0.
 
  • #3
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Mmm I haven't learnt Taylor series expansions yet. So anyway, could you tell me how to apply L'Hospital here? There are a lot of steps! I keep getting indeterminations. I cannot figure it out yet...
Thanks for the reply :)
 
  • #4
Dick
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Mmm I haven't learnt Taylor series expansions yet. So anyway, could you tell me how to apply L'Hospital here? There are a lot of steps! I keep getting indeterminations. I cannot figure it out yet...
Thanks for the reply :)
If you don't have Taylor series yet, then you'll probably want to stick with l'Hopital. But the idea is to use cos(x)=1-x^2/2!+ terms of higher order in x. It does make things easier.
 
  • #5
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Sorry but I didn't understand the idea.. could you explain the first steps of the resolution? I keep getting IND 0/0 -- I know I've got to apply l'Hopitale every time I get the indtermination but there're just too many.. it never ends.
 
  • #6
Dick
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Sorry but I didn't understand the idea.. could you explain the first steps of the resolution? I keep getting IND 0/0 -- I know I've got to apply l'Hopitale every time I get the indtermination but there're just too many.. it never ends.
l'Hopital will end at the fourth derivative. It has to. Then the denominator becomes a constant. It is a little hard to keep track of the numerator, I will admit.
 
  • #7
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Yo just plug that mofo numerator equation into WolframAlpha:
http://www.wolframalpha.com/input/?i=fourth+derivative+of+1-cos(1-cosx)

I agree, it's a nasty numerator, but you can just plug in x = 0 now. Looking at it real quick, and it's looks like the numerator at 0 equals 3, so the limit is 3.

Edit: Oh wait, the limit wouldn't be 3, it would 3/(3*4*3*2*1) = 1/24
 

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