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Lagrange Multiplier

A method for finding the maxima and minima of a function subject to constraints.

e.g. maximize f(x,y)

Subject to g(x,y)=c


We introduce a new variable (λ) called a Lagrange multiplier, and study the Lagrange function defined by


If f(x,y) is a maximum for the original constrained problem, then there exists λ such that (x,y,λ) is a stationary point for the Lagrange function (stationary points are those points where the partial derivatives of Λ are zero). not all stationary points yield a solution of the original problem.

Thus, the method of Lagrange multipliers yields a necessary condition for optimality in constrained problems.

- a powerful tool for solving this class of problems without the need to explicitly solve the conditions and use them to eliminate extra variables.