What is the Use of Mean Value Theorem for Infinity in Advanced Calculus Proof?

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Discussion Overview

The discussion revolves around the application of the Mean Value Theorem (MVT) in proving a limit related to the derivative of a function defined on the interval (0, infinity). Participants explore how to approach the problem of showing that if the limits of both f'(x) and f(x) exist and are finite, then the limit of f'(x) as x approaches infinity must be zero.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant presents the problem and expresses difficulty in using the MVT in the context of infinity.
  • Another participant questions what f(x) would look like for large x if the limit of f'(x) were not zero.
  • A different participant suggests that if the gradient were not infinitesimal, the limit would not converge to a finite value.
  • Another participant applies the MVT, stating that for a fixed a in (0, infinity), there exists a c in (a, x) such that the average rate of change equals f'(c), prompting further exploration of the implications as x approaches infinity.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are differing views on the implications of the limit of f'(x) and the application of the MVT in this context.

Contextual Notes

There are unresolved assumptions regarding the behavior of f(x) and f'(x) as x approaches infinity, as well as the specific conditions under which the MVT can be applied in this scenario.

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Here is the problem Let f be differentiable on (0,infinity) if the limit as x approaches infinity f'(x) f(x) both exist are finite prove that limit as x approaches infitity f'(x)=0.


I have trouble proving this problem I was told to use Mean Value Theorem to find a contridiction. However, I have not seen how to use the MVT when we are dealing with infinity any hints?
 
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If the limit of f'(x) was not zero, then what would f(x) look like for large x?
 
I don't know but it would be a finite number

Any help would be appreciated
 
We'll you see, if the gradient was anything other than an infinitesimal, then the limit as it approaches infinity will not converge and not be finite. Hopefully this helps.
 
Fix [itex]a \in (0, \infty)[/itex], then by the MVT there exists a [itex]c \in (a, x)[/itex] such that

[tex]\frac{f(x) - f(a)}{x - a} = f'(c)[/tex]

Now let [itex]x \rightarrow \infty[/itex]. What happens?
 

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