Tangent and Secant Lines
Move the blue slider to change the x-coordinate of point A. Watch how this affects the tangent line. Click on "Show Secant" to see the secant line through point A and a new point B. Move the green slider to change the x-coorinate of point B. What happens to the slope of the secant line as B gets closer to A from the left? from the right? Finally, you can click inside the box to change the function f(x).