The figure shows the Cartesian axes labelled x and y concentrated in the upper-right quadrant. There is an arbitrary curve labelled y equals f of x contained inside the upper-right quadrant with one local maximum point and one local minimum point and the curve gradually rising bottom left to top right. There are points a and b marked on the positive x-axis with a being closer to the origin. These points are joined by a bold line. Vertical lines join the points a and b to the curve and the area between the curve and above the positive x-axis is shaded.