Viết phân số

- Hướng dẫn giải

$$\begin{gathered} a)\frac{8}{21}=\frac{1}{3}:\frac{7}{8}=\frac{8}{7}:\frac{3}{1}=\frac{1}{7}:\frac{3}{8} \\ =\frac{2}{3}:\frac{7}{4}=\frac{4}{7}:\frac{3}{2}=\frac{2}{7}:\frac{3}{4}=\frac{4}{3}:\frac{7}{2} \end{gathered}$$

b)

$$\begin{gathered} \frac{8}{21}=\frac{-1}{3}:\frac{7}{8}=\frac{-8}{7}:\frac{-3}{1}=\frac{-1}{7}:\frac{-3}{8} \\ =\frac{-2}{3}:\frac{7}{4}=\frac{-4}{7}:\frac{3}{2} \\ =\frac{-2}{7}:\frac{-3}{4}=\frac{-4}{3}:\frac{-7}{2} \end{gathered}$$