Differentiation

differentiation, in mathematics, process of finding the derivative, or rate of change, of a function. In contrast to the abstract nature of the theory behind it, the practical technique of differentiation can be carried out by purely algebraic manipulations, using three basic derivatives, four rules of operation, and a knowledge of how to manipulate functions.

The three basic derivatives (D) are: (1) for algebraic functions, D(xⁿ) = nx^{n − 1}, in which n is any real number; (2) for trigonometric functions, D(sin x) = cos x and D(cos x) = −sin x; and (3) for exponential functions, D(e^x) = e^x.

For functions built up of combinations of these classes of functions, the theory provides the following basic rules for differentiating the sum, product, or quotient of any two functions f(x) and g(x) the derivatives of which are known (where a and b are constants): D(af + bg) = aDf + bDg (sums); D(fg) = fDg + gDf (products); and D(f/g) = (gDf − fDg)/g² (quotients).

The other basic rule, called the chain rule, provides a way to differentiate a composite function. If f(x) and g(x) are two functions, the composite function f(g(x)) is calculated for a value of x by first evaluating g(x) and then evaluating the function f at this value of g(x); for instance, if f(x) = sin x and g(x) = x², then f(g(x)) = sin x², while g(f(x)) = (sin x)². The chain rule states that the derivative of a composite function is given by a product, as D(f(g(x))) = Df(g(x)) ∙ Dg(x). In words, the first factor on the right, Df(g(x)), indicates that the derivative of Df(x) is first found as usual, and then x, wherever it occurs, is replaced by the function g(x). In the example of sin x², the rule gives the result D(sin x²) = Dsin(x²) ∙ D(x²) = (cos x²) ∙ 2x.

In the German mathematician Gottfried Wilhelm Leibniz’s notation, which uses d/dx in place of D and thus allows differentiation with respect to different variables to be made explicit, the chain rule takes the more memorable “symbolic cancellation” form: d(f(g(x)))/dx = df/dg ∙ dg/dx.