» Required Reading » Table of Contents » Chapter 6 6.1 Differentiability, the Tangent Line-Linear Approximation
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A function f of one real variable is said to be differentiable at argument x, if its graph looks like a straight line for arguments in any open interval including x. (an open interval is one that does not contain its endpoints). Its derivative at x is the slope of that line. (to be more precise, for whatever positive criterion of nearness you choose however small, there is an open interval containing x so that for every x' in the interval other than x itself, the difference between and the slope of that line is less than that criterion.) Note
The line that f resembles near argument x is called the tangent line to f at argument x and and the linear function it represents is called the linear approximation to f at argument x. The slope of the tangent line at x is given by df / dx on that line for any two point P₂ and P₁ on it, with

df = P_2f - P_1f , dx = P_2x - P_1x

We use the notation dx and df to denote changes in the corresponding variables that are so small that we can assume the linear approximation to f (and to any other function involved in the definition of f) is exacty satisfied (and if there is no such distance create one in your imagination). Changes of this sort are called differentials.  The derivative of  f  at argument x is usually written as In fact if df and dx are differentials, then the derivative is by definition since the derivative is the ratio of the change of f to the change of x in the linear approximation.