Explain why a positive derivative on an interval impliesthat the function is increasing on the interval.

Calculus: Early Transcendentals
8th Edition
ISBN:9781285741550
Author:James Stewart
Publisher:James Stewart
Chapter1: Functions And Models
Section: Chapter Questions
Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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Explain why a positive derivative on an interval implies
that the function is increasing on the interval.

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Step 1

On an open interval

Suppose f is a function on an open interval I that may be infinite in one or both directions (i..e, I is of the form \! (a,b)(a,\infty)(-\infty,b), or (-\infty,\infty)). Suppose the Derivative of f exists and is positive everywhere on I, i.e., \! f'(x) > 0 for all x \in I. Then, f is an increasing function on I, i.e.:

x_1,x_2 \in I, x_1 < x_2 \implies f(x_1) < f(x_2)

On a general interval

Suppose f is a function on an interval I that may be infinite in one or both directions and may be open or closed at either end. Suppose f is a continuous function on all of I and that the derivative of f exists and is positive everywhere on the interior of I, i.e., f'(x) > 0 for all x \in I other than the endpoints of I (if they exist). Then, f is an increasing function on I, i.e.:

x_1,x_2 \in I, x_1 < x_2 \implies f(x_1) < f(x_2)

 

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