Изменения

<tex>\Delta _x \Delta _y f=\frac {\partial^2 f}{\partial x \partial y}(x+\theta_1\Delta x,y+\theta_2 \Delta y) \Delta x \Delta y</tex>

<tex>\Delta _y \Delta _x f=\frac {\partial^2 f}{\partial y \partial x}(x+\theta_3\Delta x,y+\theta_4 \Delta y) \Delta x \Delta y</tex>