What is the Proof for Rolle's Theorem Using the Intermediate Value Theorem?

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Homework Help Overview

The discussion revolves around proving the existence of a point \( c \) within the interval [0,3] such that \( f(c) = c \) for a function that is continuous on that interval. The original poster presents values of the function at specific points and references Rolle's Theorem in their reasoning.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of the function's values and continuity. The original poster attempts to apply Rolle's Theorem but faces challenges in connecting it to the desired result. Others suggest defining a new function \( g(x) = f(x) - x \) to analyze the problem using the Intermediate Value Theorem.

Discussion Status

The discussion is active, with participants questioning the necessity of Rolle's Theorem and clarifying the requirements of the problem. Some guidance has been offered regarding the use of the Intermediate Value Theorem, and there is an acknowledgment of the relevance of continuity over differentiability.

Contextual Notes

There is a note that the function does not need to be differentiable, only continuous, and that some provided information may be unnecessary for solving the problem.

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Homework Statement


A function is continuous/differentiable on the interval [0,3]. I have been given that f(0)=1, f(1)=2, f(3)=2.
I need to prove that there exists a c within the interval [0,3] such that f(c) = c.

Homework Equations



The Attempt at a Solution


f(1)=f(3)=2. From Rolle's theorem, there exists a c within [1,3] such that f`(c)=0. From this I am able to show that at some point c within [1,3] the function must be constant i.e f(c)=k.
I am not sure how to show that the constant is equal to the point i.e k=c.
 
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The problem does not ask you about the derivative of f so Rolle's theorem is unnecessary. Also it makes no sense to say that a function is constant at a point! Every function has a single value at every point of its domain.

Define g(x)= f(x)- x. g(0)= 1- 0= 0, g(1)= 2- 1= 1, and g(3)= 2- 3= -1. That is, g(1)> 0 and g(3)< 0. What does that tell you?


(Much of the information given in this problem is unnecessary! The function does not have to be differentiable, just continuous. The information about f(0)= 1 is irrelevant. And, in fact, the desired point must be in the interval [1, 3].)
 
HallsofIvy said:
Define g(x)= f(x)- x. g(0)= 1- 0= 0, g(1)= 2- 1= 1, and g(3)= 2- 3= -1. That is, g(1)> 0 and g(3)< 0. What does that tell you?

Thanks for the response. This helped a lot.
 
Last edited:

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