a) Ta có: \(\dfrac{3}{7}=\dfrac{3 \times 5}{7 \times 5}=\dfrac{15}{35}\); \(\dfrac{2}{5}=\dfrac{2 \times 7}{5 \times 7}= \dfrac{14}{35}\).
Vì \(\dfrac{15}{35}>\dfrac{14}{35}\) nên \(\dfrac{3}{7}>\dfrac{2}{5}\).
b) Ta có \(\dfrac{5}{9}<\dfrac{5}{8}\) (vì \(9>8\)).
c) \(\dfrac{8}{7}=\dfrac{8 \times 8}{7 \times 8}=\dfrac{64}{56}\); \(\dfrac{7}{8}=\dfrac{7 \times 7}{8 \times7}=\dfrac{49}{56}\).
Vì \(\dfrac{64}{56}>\dfrac{49}{56}\) nên \(\dfrac{8}{7}>\dfrac{7}{8}\).
Cách khác: Vì \(\dfrac{8}{7}> 1 \) ; \(\dfrac{7}{8}< 1\) nên \(\dfrac{8}{7}\) > \(\dfrac{7}{8}\).
