The Mixed partial Derivatives Are Equal
It is not an accident that the estimates for fxy(1,2)and fyx (1,2)are equal in Example 2, because the same values of function are used to calculate each one. The fact that fxy=fyx Examples 1and 2 corroborates the following general result.
Most of the function we will encounter not only have fxy and fyx continuous,but all their higher order partial derivatives (such as fxxy or fxyyy)will exist and be continuous.we call such functions smooth.