Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: https://www.khanacademy.org/math/multivariable-calculus/applications-of-multivariable-derivatives/optimizing-multivariable-functions-videos/v/second-partial-derivative-test How to determine if the critical point of a two-variable function is a local minimum, a local maximum, or a saddle point.