The rate of change of most functions varies from point to point. [br]One can find out how the function f(x) changes by comparing it [br]to the same function displaced a bit, say Δx.[br][br]The difference function Δf = f(x + Δx) - f(x) gives us a [br]rough sense of how the function varies from point to point.[br][br]This rough sense is refined by making the displacement smaller. [br][br]The trouble is that the difference function gets smaller and smaller [br]as the displacement gets smaller and smaller. [br][br]The situation can be saved by dividing the shrinking difference function [br]by something else that is shrinking at the same time, [br]i.e., the value of the displacement Δx.[br][br]You can enter any function that depends of up to 3 parameters a,b, and c.[br][br]Challenge:[br][br]Why does a WARNING! appear on the screen when the function Δf / Δx is displayed?