[TEX]\frac{3}{2.5} + \frac{3}{5.8} + \frac{3}{8.11} + ... + \frac{3}{67.70}[/TEX]
[TEX]= \frac{1}{2} - \frac{1}{5} + \frac{1}{5} - \frac{1}{8} + \frac{1}{8} - \frac{1}{11} + \frac{1}{11} - .... + \frac{1}{67} - \frac{1}{70}[/TEX]
[TEX]= \frac{1}{2} + (\frac{1}{5} - \frac{1}{5}) + (\frac{1}{8} - \frac{1}{8}) + (\frac{1}{11} - \frac{1}{11}) + ... + (\frac{1}{67} - \frac{1}{67}) - \frac{1}{70}[/TEX]
[TEX]= \frac{1}{2} - \frac{1}{70}[/TEX]
[TEX]= \frac{17}{35}[/TEX]