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Homework Help: Prove a limit using the mean value theorem

  1. Aug 18, 2012 #1
    I am supposed to use the mean-value theorem to show that lim_x→infty(√(x+5)-√(x))=0.

    Can anyone help me solving this problem?

    I have tried to set up the mean value theorem, but i just do not know how to proceed.
  2. jcsd
  3. Aug 18, 2012 #2
    It is rather obvious even without the mean value theorem. Just multiply and divide by the conjugate.
  4. Aug 18, 2012 #3
    I also believe that the limit is obvious, but the exercise here is to prove it using the mean value theorem.
  5. Aug 18, 2012 #4


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    The "mean value theorem" says that, for f continuous on [a, b] and differentiable on (a, b), there exist c between a and b such that
    [tex]\frac{f(b)- f(a)}{b- a}= f'(c)[/tex].

    Take f(x)= x1/2, b= x+5, a= x. Of course, as x goes to infinity, so do both x and x+5 so a number "between them", c, must also go to infinity. What can you say about f'(c) as c goes to infinity?
  6. Aug 18, 2012 #5
    So, if I write that ((f(b)-f(a))/b-a)=f'(c), and takes the value as you say. I know that f'(x)=1/2sqrt(x). The limit of this when x goes to infinity, is 1 divided by an infinity large number which is 0.

    Am I right with my reasoning below: this proves my limit, since lim(f(b)-f(a))=lim(f'(c)*(b-a))=0 since f'(c)=0?
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