D^3f(x_0,y_0) ((x,y)-(x_0, y_0), (x,y)-(x_0, y_0), (x,y)-(x_0, y_0)) = \\
= D^3f(x_0,y_0)((x-x_0, y-y_0), (x-x_0, y-y_0), (x-x_0, y-y_0)) \\ \\
= \partial xxx f(x_0, y_0) (x-x_0)^3 + 3 \partial xxy f(x_0, y_0) (x-x_0)^2 (y-y_0) \\ + 3 \partial yyx f(x_0, y_0) (x-x_0) (y-y_0)^2 + \partial yyy f(x_0, y_0) (y-y_0)^3