Multivariable Analysis (Reprint)

Let PI' P2 be two points near Po such that Po, PI, P2 are not on a straight line. The linear function of x and y whose values at Po, PI' P2 are equal to those off at these points approximates f near Po; The derivatives off at Po are defined to be the limits of these ratios as PI and P2 approach Po (subject to an important regularity condition).