$\frac{1}{1.2}$ + $\frac{1}{2.3}$ + $\frac{1}{3.4}$ + ... + $\frac{1}{8.9}$
= (1 - $\frac{1}{2}$) + ($\frac{1}{2}$ - $\frac{1}{3}$) + ($\frac{1}{3}$ - $\frac{1}{4}$) + ... + ($\frac{1}{8}$ - $\frac{1}{9}$)
= 1 - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + ... + $\frac{1}{8}$ - $\frac{1}{9}$
= 1 - $\frac{1}{9}$
= $\frac{8}{9}$
Chúc học tốt!!!