\lim_{n\to \infty}\sum_{i=1}^{n}\sqrt{(\Delta x_i)^{2}+ [f'(x_i^{*})\cdot \Delta x_i]^{2}} = \lim_{max \Delta x_i \to 0} (\sum_{i=1}^{n})\sqrt{1+ [f'(x_i^{*})]^{2}\cdot \Delta x}\