Derivative as slope of the tangent

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