Points A and B are points on the curve. A is fixed, while B is not.
Start by moving B towards A (click on B and drag towards A), from both directions left and right.
What happens to the secant AB as B approaches A?
Check both check boxes. Repeat the process.
What happens to the secant AB as B approaches A?
What happens to ? ? The slope of the secant AB? Compare it to the slope of the tangent at A