Drag the point B (x, f(x)) to see how a Secant Becomes a Tangent [br][br]Use the check boxes to see the slopes of the Secant line and of the Tangent line

This applte shows how a Secant Becomes a Tangent and it shows the slopes of the Secant line and of the Tangent line , thus establishing a correspondence between differentiability geometrically and algebraically.