How can you show that $\displaystyle \int\int\frac{\vartheta^{2}F(x,y)}{\vartheta x \vartheta y}$ $\displaystyle dx dy$ over the rectangle $\displaystyle x_{0}\le x \le x_{1}$, $\displaystyle y_{0}\le y \le y_{1}$, is $\displaystyle F(x_{1},y_{1})-F(x_{0},y_{1})-$$\displaystyle F(x_{1},y_{0})+F(x_{0},y_{0})$.