In this applet, we see a function `f` graphed in the `xy`-plane. You can move the blue
point on the `x`-axis and you can change `delta`, the "radius" of an interval centered
about that point. The point has `x`-value `c`, and you can see the values of
`c` and `f(c)`. You can type in your own functions in the left input box, or
you can use the pre-loaded examples in the right drop down box.