Bài 2:
a) \(7.\left(-28\right)=\left(-49\right).4\)
⇒ (1) \(\dfrac{7}{4}=\dfrac{-49}{-28}\) ⇔ \(\dfrac{7}{4}=\dfrac{49}{28}\)
(2) \(\dfrac{7}{-49}=\dfrac{4}{-28}\)
(3) \(\dfrac{4}{7}=\dfrac{-28}{-49}=\dfrac{28}{49}\)
(4) \(\dfrac{-28}{4}=\dfrac{-49}{7}\)