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/quadratic-approximations/v/what-do-quadratic-approximations-look-like After learning about local linearizations of multivariable functions, the next step is to understand how to approximate a function even more closely with a quadratic approximation.