Đáp án: $\frac{9}{10}$
Giải thích các bước giải:
$\frac{1}{2}$ + $\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{20}$ +$\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ + $\frac{1}{72}$ + $\frac{1}{90}$
= $\frac{1}{1.2}$ + $\frac{1}{2.3}$ + $\frac{1}{3.4}$ + $\frac{1}{4.5}$ + $\frac{1}{5.6}$ + $\frac{1}{6.7}$ + $\frac{1}{7.8}$ + $\frac{1}{8.9}$ + $\frac{1}{9.10}$
= 1 - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{8}$ + $\frac{1}{8}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{10}$
= $\frac{1}{1}$ - $\frac{1}{10}$
= $\frac{10}{10}$ - $\frac{1}{10}$
= $\frac{9}{10}$
