Secant&Tangent Applet

Drag the red "beads" and note the display of rise, run, and slope.
Alt-click on the secant line toggles extension on and off.
The recenter button puts the point of tangency at the centre of the graph window.

Leaving the bead on the tangent fixed and dragging the other allows the tangent slope to be estimated by making the secant very short.

[If your browser refuses to run this app in an iFrame then you might try to run it in a new "insecure" window ]