D f(x,y) (x_0,y_0) = \triangledown f(x,y) \cdot \left(\begin{array}{ccc} x_0^1 \\ \cdot \\ \cdot \\ \cdot \\x_0^n \\ y_0^1 \\ y_0^2 \\ y_0^3 \end{array}\right)