Graphing the Derivative
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Calculus Applied to the Real World

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To begin, we recall two basic facts about the derivative $f'(x)$ of a function $f(x):$

The graph of the derivative function $f'(x)$ gives us interesting information about the original function $f(x).$ The following example shows us how to sketch the graph of $f'(x)$ from a knowledge of the graph of $f(x).$

Example 1 Sketching the Graph of the Derivative

Let $f(x)$ have the graph shown below.

Give a rough sketch of the graph of $f'(x).$

Solution

Remember that $f'(x)$ is the slope of the tangent at the point $(x, f(x))$ on the graph of $f.$ To sketch the graph of $f',$ we make a table with several values of $x$ (the corresponding points are shown on the graph) and rough estimates of the slope of the tangent $f'(x).$

$x$$0$$0.5$$1$$1.5$$2$$2.5$$3$
$f'(x)$$3$$0$$-4$$-3$$0$$1$$0$

(Note that rough estimates are the best we can do; it is difficult to measure the slope of the tangent accurately without using a grid and a ruler, so we couldn't reasonably expect two people's estimates to agree. However, all that is asked for is a rough sketch of the derivative.) Plotting these points suggests the curve shown below.

Notice that the graph $f'(x)$ intersects the $x$-axis at points that correspond to the high and low points on the graph of $f(x).$ Why is this so?

Here is a more interactive example.

Example 2 Graph of Derivative

Let $f(x)$ have the graph shown below.

Complete the following table, giving rough estimates of the slope of the tangent $f'(x)$ at the given values of $x.$

$x$$-3$$-2$$-1$$0$$1$$2$$3$
$f'(x)$
Peek at Answers






       

Now plot these points, and hence make a rough sketch of the graph of $f'(x).$ Which of the following best approximates your sketch of the graph of $f'(x)$? (click on one)




 

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Index of On-Line Topics
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Utility: Function Evaluator & Grapher

Last Updated:November, 1997
Copyright © 1996 StefanWaner and Steven R. Costenoble