Bài 6 (1,0 điểm) Tính nhanh S= $\frac{1}{1.2}$+ $\frac{1}{2.3}$+ $\frac{1}{3.4}$+...+ $\frac{1}{100.101}$ 

$\text{ S = $\frac{1}{1.2}$ + $\frac{1}{2.3}$ + $\frac{1}{3.4}$ + ... + $\frac{1}{100.101}$ }$

$\text{S = $\frac{1}{1}$ - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + ... +$\frac{1}{100}$ - $\frac{1}{101}$ }$

$\text{ S = $\frac{1}{1}$ - $\frac{1}{101}$}$

$\text{ S = $\frac{100}{101}$}$