a) \(\dfrac{3}{4} = \dfrac{21}{28} \) ; \(\dfrac{5}{7} = \dfrac{20}{28} \)
Mà \(\dfrac{21}{28} > \dfrac{20}{28} \) (vì \(21>20\))
Vậy \(\dfrac{3}{4} > \dfrac{5}{7} \).
b) \(\dfrac{2}{7} = \dfrac{4}{14} \) ; Giữ nguyên \(\dfrac{4}{9} \)
Mà \(\dfrac{4}{14} < \dfrac{4}{9} \) (vì \(14>9\))
Vậy \( \dfrac{2}{7} < \dfrac{4}{9}\);
c) \(\dfrac{5}{8}< 1; \quad \dfrac{8}{5}> 1\) nên ta có \(\dfrac{5}{8} <1< \dfrac{8}{5} \)
Vậy \( \dfrac{5}{8}\) \( <\dfrac{8}{5}\).