For the sake of argument, suppose that f(a) is smaller than f(b). Then f can't increase from f(a) to f(b) unless at some points it has slope at least as great as the slope of the straight line joining the point (a,f(a)) to the point (b,f(b)).

Draw a picture to show a possible curve for f, together with this straight line

What is the slope of the straight line?

Likewise, if f is to increase from f(a) to f(b) over the interval, then its slope can't always be greater than the slope of the straight line joining (a,f(a)) to the point (b,f(b)).

It's tempting to use this argument to conclude at once that since f' has to be bigger that than the slope of the straight line joining (a,f(a)) to (b,f(b)) at some points, and less than it at others, it must be exactly equal to this slope at some point.

What's the gap in this argument?

What theorem might you want to apply to fill that gap?

Why can't you?

Analysis WebNotes by John Lindsay Orr.
Comments to the author: jorr@math.unl.edu

All contents copyright (C) 1996 John L. Orr
University of Nebraska--Lincoln
All rights reserved

Last modified: May 1996