A very typical calculus problem is given the equation of a function, to find information about it (extreme values, concavity, increasing, decreasing, etc., etc.). This is usually done by computing and analyzing the first derivative and the second derivative. All the textbooks show how to do this with copious examples and exercises. I have nothing to add to that. One of the “tools” of this approach is to draw a number line and mark the information about the function and the derivative on it.
A very typical AP Calculus exam problem is given the graph of the derivative of a function, but not the equation of either the derivative or the function, to find all the same information about the function. For some reason, students find this difficult even though the two-dimensional graph of the derivative gives all the same information as the number line graph and, in fact, a lot more.
Looking at the graph of the derivative in the x,y-plane it is easy to determine the important information. Here is a summary relating the features of the graph of the derivative with the graph of the function.
|> 0||is increasing|
|< 0||is decreasing|
|changes – to +||has a local minimum|
|changes + to –||has a local maximum|
|increasing||is concave up|
|decreasing||is concave down|
|extreme value||has a point of inflection|
Here’s a typical graph of a derivative with the first derivative features marked.
Here is the same graph with the second derivative features marked.
The AP Calculus Exams also ask students to “Justify Your Answer.” The table above, with the columns switched does that. The justifications must be related to the given derivative, so a typical justification might read, “The function has a relative maximum at x = -2 because its derivative changes from positive to negative at x = -2.”
|y is increasing||> 0|
|y is decreasing||< 0|
|y has a local minimum||changes – to +|
|y has a local maximum||changes + to –|
|y is concave up||increasing|
|y is concave down||decreasing|
|y has a point of inflection||extreme values|
Originally posted on October 26, 2012, and my single most viewed post over the years.