Accumulation 4: Graphing Ideas in Accumulation – Concavity
In the last post we saw how thinking about Riemann sum rectangles, RΣR, moving across the graph of the derivative made it easy to see when the function whose derivative was given increased and decreased and had its local extreme values. Today we will consider concavity.
Suppose a derivative is constant, its graph a horizontal line. In this case each successive RΣR is exactly the same size and adds exactly the same amount to the accumulated function. The function’s graph increases (or decreases) by exactly the same amount – it is linear.
For derivatives that are not constant, the change in the resulting function is not constant and the function’s graph bends up or down. This bending of the function is referred to as its concavity. If it bends up, the function is increasing faster and its graph is concave up; if it bends down, it is increasing slower (or decreasing faster) and concave down.
The graph above shows pairs of RΣR in different intervals as they move along the graph of a derivative. Consider the dark blue rectangle to be the previous position of the red rectangle.
As the RΣR moves from a to b each red rectangle is larger than the dark blue one. Each move adds more to the accumulated sum than the previous one. The graph of the function increases more with each move – it is concave up.
As the RΣR move from b to d (there are two pairs drawn) each red RΣR has a smaller value than the previous one. (Remember when they are below the x-axis the longer (red) RΣR has a smaller value.) In the interval [b, d] less is added to the accumulated sum (or more is subtracted) with each move to the right. Therefore, the graph of the function bends down – the function is concave down.
In the last section, from d to f the red rectangle now has a larger value than the dark blue one. (Again remember that when the have negative values, the shorter rectangle, has the larger value.) The graph of the function again bends up – is concave up.
Putting these ideas together with those in the last post we can see how the moving RΣR idea can distinguish the four shapes of the graph of the accumulating function:
- On [a, b] the function’s graph is increasing and concave up; the RΣR are positive and getting more positive (longer).
- On [b, c] the function’s graph is increasing and concave down; the RΣR are positive and getting less positive (shorter).
- On [c, d] the function’s graph is decreasing and concave down; the RΣR are negative and getting more negative (longer).
- On [d, e] the function’s graph is decreasing and concave up; the RΣR are negative and getting less negative (shorter).
- At the extreme values of the derivative, the concavity of the function changes from up to down or down to up. These are called points of inflection.
Questions in which students are asked about the properties of a function given the graph, but not the equation, of the derivative are very common. Many students (including me) find this approach easier and more intuitive than working strictly with derivative ideas.