Giải thích các bước giải:
$B = \dfrac{1}{30} + \dfrac{1}{42} + \dfrac{1}{56} + \dfrac{1}{72} + \dfrac{1}{90} + \dfrac{1}{110} + \dfrac{1}{132}$
$= \dfrac{1}{5.6} + \dfrac{1}{6.7} + \dfrac{1}{7.8} + \dfrac{1}{8.9} + \dfrac{1}{9.10} + \dfrac{1}{10.11} + \dfrac{1}{11.12}$
$= \dfrac{1}{5} - \dfrac{1}{6} + \dfrac{1}{6} - \dfrac{1}{7} + \dfrac{1}{7} - \dfrac{1}{8} + \dfrac{1}{8} - \dfrac{1}{9} + \dfrac{1}{9} - \dfrac{1}{10} + \dfrac{1}{10} - \dfrac{1}{11} + \dfrac{1}{11} - \dfrac{1}{12}$
$= \dfrac{1}{5} - \dfrac{1}{12}$
$= \dfrac{7}{60}$
