Bài $31$.
$a$) $\dfrac{2 - \dfrac{2}{3} + \dfrac{2}{7}}{5 - \dfrac{5}{3} + \dfrac{5}{7}}$
$= \dfrac{2(1 - \dfrac{1}{3} + \dfrac{1}{7})}{5(1 - \dfrac{1}{3} + \dfrac{1}{7}}$
$= \dfrac{2}{5}$
$b$) $\dfrac{\dfrac{2}{3} + \dfrac{2}{7} - \dfrac{1}{14}}{1 + \dfrac{3}{7} - \dfrac{3}{28}}$
$= \dfrac{\dfrac{2}{3} + \dfrac{2}{7} - \dfrac{2}{28}}{\dfrac{3}{3}+ \dfrac{3}{7} - \dfrac{3}{28}}$
$= \dfrac{2(\dfrac{1}{3} + \dfrac{1}{7} - \dfrac{1}{18})}{3(\dfrac{1}{3} + \dfrac{1}{7} - \dfrac{1}{18})}$
$= \dfrac{2}{3}$
$c$) $\dfrac{12 - \dfrac{12}{7} - \dfrac{12}{289} - \dfrac{12}{85}}{4 - \dfrac{4}{7} - \dfrac{4}{289} - \dfrac{4}{85}}$
$= \dfrac{12(1- \dfrac{1}{7} - \dfrac{1}{289} - \dfrac{1}{85})}{4(1 - \dfrac{1}{7} - \dfrac{1}{289} - \dfrac{1}{85}}$
$= \dfrac{12}{4}$
$= 3$
$d$) $\dfrac{\dfrac{3}{5} + \dfrac{3}{13} + \dfrac{3}{169} + \dfrac{1}{27}}{1 + \dfrac{5}{13} + \dfrac{5}{169} + \dfrac{5}{81}}$
$= \dfrac{\dfrac{3}{5} + \dfrac{3}{13} + \dfrac{3}{169} + \dfrac{3}{81}}{\dfrac{5}{5} + \dfrac{5}{13} + \dfrac{5}{169} + \dfrac{5}{81}}$
$= \dfrac{3(\dfrac{1}{5} + \dfrac{1}{13} + \dfrac{1}{169} + \dfrac{1}{81})}{5(\dfrac{1}{5} + \dfrac{1}{13} + \dfrac{1}{169} + \dfrac{1}{81})}$
$= \dfrac{3}{5}$
