In analysis of functions of one variable, the differentiability of the function at a given point is equivalent to having a finite derivative at this point.

Below we consider in detail the case of a function of two variables. Then, there is usually just one restriction, so the formulation of the problem looks as follows

z=f(x, y)›extr
g(x,y) = 0

Here z = f (x, y) is often referred as the objective function, and g (x, y) = 0 is a constraint.

The simplest approach to the problem is possible when one of the variables may be expressed in terms of the other from the constraint, then the problem reduces to standard extrema problem for a function of one variable.