This applet will allow you to explore secant and tangent lines. Type whatever function you want into the box for f(x). Use the ^ key to enter in exponents just like you would on most graphing calculators. It works best if you use a function that fits in the given viewing window. If you have to change the viewing window, some of the controls will move too.
If you click on the box next to Secant Line, a red secant line that connects two points on the graph of the f(x) will appear along with the equation of that line. The sliders c & d can be used to change the two points.
If you click on the box next to Tangent Line, a blue tangent line will appear along with its equation. The slider c can be used to move the point at which the line is tangent to the graph.