Show by means of example that lim/x→a/[f(x)g(x)] may exist

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Homework Help Overview

The discussion revolves around the limit of the product of two functions, specifically exploring the scenario where the limit of the product exists even though the individual limits of the functions do not. The subject area pertains to calculus and limit theory.

Discussion Character

  • Exploratory, Conceptual clarification

Approaches and Questions Raised

  • Participants are attempting to find examples, particularly using piecewise and rational functions, to illustrate the statement. Questions arise regarding the definitions and behaviors of the functions involved, with some participants suggesting specific function forms.

Discussion Status

The discussion is ongoing, with participants providing examples and engaging in a back-and-forth about the nature of the functions. Some guidance has been offered in the form of specific function suggestions, but there is no explicit consensus on a valid example yet.

Contextual Notes

Participants are working under the constraints of demonstrating the limit behavior without providing complete solutions. There is mention of a corollary that adds complexity to the discussion, highlighting the nuanced nature of the functions being considered.

MathewsMD
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Show by means of example that lim/x→a/[f(x)g(x)] may exist even though neither lim/x→a/f(x) nor lim/x→a/g(x) exists.

I have tried using examples such as piecewise functions and rational functions, but can never validate the statement.

Any guidance and help would be great.

Thanks.
 
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Let f(x) be any crazy function. Let g(x) = 1/f(x).
 
mathman said:
Let f(x) be any crazy function. Let g(x) = 1/f(x).

You forgot to insert Arildno's corollary:
"Let f(x) be any crazy function. Let g(x) = 1/f(x). THEN, g(x) is most likely also a crazy function"

Not very useful in this context, of course, but the result is beautiful, nonetheless. :smile:
 
MathewsMD said:
Show by means of example that lim/x→a/[f(x)g(x)] may exist even though neither lim/x→a/f(x) nor lim/x→a/g(x) exists.

I have tried using examples such as piecewise functions and rational functions, but can never validate the statement.

Any guidance and help would be great.

Thanks.

Consider the function which is equal to 1 if its argument is rational and 0 otherwise.
 
pasmith said:
Consider the function which is equal to 1 if its argument is rational and 0 otherwise.
I presume you mean "let f(x)= 1 if x is rational, 0 if x is irrational.

And then let g(x)= 0 if x is rational, 1 if x is irrational.


fg(x)= 0 for all x so it trivially differentiable.
 

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