Figure 3

Triangularly divided areas in the case of n = 3.

The area of P₀P₁P₂P₃ is$\begin{array}{cc}\hfill {\sigma }_3& =R_{3.0}-R_{3.1}-R_{3.2}-R_{3.3}\hfill \\ =\frac{1}{2}\left(\Delta x_2\Delta y_1-\Delta y_2\Delta x_1\right)+\frac{1}{2}\left(\Delta x_3\Delta y_1-\Delta y_3\Delta x_1\right)+\frac{1}{2}\left(\Delta x_3\Delta y_2-\Delta y_3\Delta x_2\right).\hfill \end{array}$