Area of the region between the graphs of two functions

The applet helps you understand how the integrals can be used to find the area of the region enclosed by the graphs of two functions over a given interval. Try for a more interesting example.
When you press "Enable shift" button, you will have access to a constant D that can be used to shift up or down both graphs. In that way you can see why the area is equal to the integral of f - g, even in the case when the functions are not positive on [a, b].