## Introduction

A function f(x) is differentiable at the point x_{0} if ∃ f' such that for all ε > 0 sufficiently small, there exists a δ > 0 such that ∀ x with | x - x_{0} | < δ then

| ((f(x) - f(x

_{0})) / (x - x_{0})) - f' | < εThe value of f' is called the derivative of f at the point x_{0}.