Question

Rút gọn các phân số sau: a) \(\dfrac{-21}{28}\); \(\dfrac{-39}{52}\); \(\dfrac{-24}{63}\); \(\dfrac{12}{-21}\)               b, \(\dfrac{-270}{450}\); \(\dfrac{11}{-143}\); \(\dfrac{-132}{156}\); \(\dfrac{-26}{169}\)