Bài làm:
$\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ + $\frac{1}{72}$ +$\frac{1}{90}$
= $\frac{1}{5.6}$ + $\frac{1}{6.7}$ + $\frac{1}{7.8}$ + $\frac{1}{8.9}$ + $\frac{1}{9.10}$
=$\frac{1}{5}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{8}$ +$\frac{1}{8}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{10}$
=$\frac{1}{5}$ - $\frac{1}{10}$ = $\frac{1}{10}$
*Chúc học tốt*