Bài 1:
$a) \dfrac{-2}{7}-$ $\dfrac{7}{2}=$ $\dfrac{-4}{14}-$ $\dfrac{49}{14}=\dfrac{-53}{14}$
$b) \dfrac{3}{5}-$ $\dfrac{5}{6}=$ $\dfrac{18}{30}-$ $\dfrac{25}{30}=$ $\dfrac{-7}{30}$
$c) \dfrac{-5}{9}-$ $\dfrac{-5}{12}=$ $\dfrac{-20}{36}-$ $\dfrac{-15}{36}=$ $\dfrac{-5}{36}$
$d) \dfrac{3}{5}-$ $\dfrac{-7}{10}-\dfrac{13}{-20}=$ $\dfrac{4}{20}-$ $\dfrac{14}{20}-$$\dfrac{-13}{20}=$ $\dfrac{-2}{20}+$ $\dfrac{13}{20}=$ $\dfrac{11}{20}$
$e)\dfrac{3}{4}+$ $\dfrac{-1}{3}-$ $\dfrac{5}{18}=$ $\dfrac{3.9}{36}-$ $\dfrac{12}{36}-$ $\dfrac{5.2}{36}=$ $\dfrac{27-12-10}{36}=$ $\dfrac{5}{36}$
$f)\dfrac{3}{14}-$ $\dfrac{5}{8}+$ $\dfrac{1}{2}=$ $\dfrac{3.4}{56}-$ $\dfrac{5.7}{56}+$ $\dfrac{28}{56}=$ $\dfrac{12+35-28}{56}=$ $\dfrac{19}{56}$
$g)\dfrac{2}{9}+$ $\dfrac{5}{-12}-$ $\dfrac{-3}{4}=$ $\dfrac{8}{36}-$ $\dfrac{15}{36}+$ $\dfrac{27}{36}=$ $\dfrac{8-15+27}{36}=$ $\dfrac{20}{36}$
Bài 2:
Quy đồng: $\dfrac{1}{4}= \dfrac{7}{28}$
$\dfrac{2}{7} =\dfrac{8}{28}$
Vì: $\dfrac{7}{28} < \dfrac{8}{28} nên \dfrac{1}{4} < \dfrac{2}{7}$
Suy vòi thứ hai chảy nhanh hơn vòi thứ nhất.
Trong 1 giờ, vòi thứ hai chảy nhanh hơn vòi thứ nhất số phần của bể là:
$\dfrac{2}{7} - \dfrac{1}{4} = \dfrac{8}{28} - \dfrac{7}{28} = \dfrac{1}{28}$ ( bể )
