This Demonstration shows the minimization or maximization of a function

(blue) subject to a constraint

(red). The green point shows the optimal solution.
The graphic on the left shows the level curves of

and

together with the gradients. On the right, a 3D graphic of the function is shown together with the constraint of

projected onto the surface of

.
For either the minimum or maximum, the gradients of the function and the constraint are parallel:

, with the Lagrange multiplier

. By moving the point around the plot region, you can see that this is a necessary condition for constrained optimization.