$\frac{2}{3.5}$ + $\frac{2}{5.7}$ + ... + $\frac{2}{99.101}$
Ta thấy :
$\frac{2}{3.5}$ = $\frac{1}{3}$ - $\frac{1}{5}$.
$\frac{2}{5.7}$ = $\frac{1}{5}$ - $\frac{1}{7}$
.......................
$\frac{2}{99.101}$ = $\frac{1}{99}$ - $\frac{1}{101}$
= $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{7}$ + ... + $\frac{1}{99}$ - $\frac{1}{101}$
= $\frac{1}{3}$ - $\frac{1}{101}$
= $\frac{98}{103}$.
