Quy đồng mẫu số các phân số sau: a) -2/21; 5/-24 b) -7/12; 11/-18

* Hướng dẫn giải

a) 

$$\begin{gathered} \begin{array}{l}\frac{-2}{21};\frac{5}{-24}\\ MC=168\end{array} \\ \begin{array}{l}\frac{-2}{21}=\frac{-\mathrm{2.8.}}{21.8}=\frac{-16}{168}\\ \frac{5}{-24}=\frac{-5}{24}=\frac{-5.7}{24.7}=\frac{-35}{168}\end{array} \end{gathered}$$

b) 

$$\begin{gathered} \begin{array}{l}\frac{-7}{12};\frac{11}{-18}\\ MC=36\end{array} \\ \begin{array}{l}\frac{-7}{12}=\frac{-7.3}{12.3}=\frac{-21}{36}\\ \frac{11}{-18}=\frac{-11}{18}=\frac{-11.2}{18.2}=\frac{-22}{36}\end{array} \end{gathered}$$