Cópia de Definition of derivative
Click on the forward button to see the various components for the construction of the derivative, and observe that the ratio cannot be computed when . Then take the value of to zero by using the slider, and see what happens to the limit of the Newton quotient, which is well defined.
You can experiment with derivatives at different points on the curve by resetting the applet and by moving the starting point along the curvbe before clicking the forward button