$\begin{array}{l}a)\ \text{- Ta có : $\dfrac{28}{-49}=\dfrac{-4}7$}\\\dfrac{-6}7<\dfrac{-4}7\\\to \dfrac{-6}7<\dfrac{28}{-49}\\\,\\b)\ \rm BCNN(7,9)=63\\\to \begin{cases} \dfrac{-3}7=\dfrac{-3.9}{7.9}=\dfrac{-27}{63}\\\dfrac{-4}9=\dfrac{-4.7}{9.7}=\dfrac{-28}{63}\end{cases}\\\dfrac{-27}{63}>\dfrac{-28}{63}\\\to \dfrac{-3}7>\dfrac{-4}9\\\,\\c)\ \text{- Ta có : $\dfrac{10}{15}=\dfrac23\\\dfrac{12}{16}=\dfrac34$}\\\rm BCNN(3,4)=12\\\to\begin{cases} \dfrac23=\dfrac{2.4}{3.4}=\dfrac{8}{12}\\\dfrac34=\dfrac{3.3}{4.3}=\dfrac9{12}\end{cases}\\\dfrac8{12}<\dfrac9{12}\\\to \dfrac{10}{15}<\dfrac{12}{16}\\\,\\d)\ \rm BCNN(18,21)=126\\\to\begin{cases} \dfrac{11}{18}=\dfrac{11.7}{18.7}=\dfrac{77}{126}\\\dfrac{17}{21}=\dfrac{17.6}{21.6}=\dfrac{102}{126}\end{cases}\\\dfrac{77}{126}<\dfrac{102}{126}\\\to\dfrac{11}{18}<\dfrac{17}{21} \end{array}$
