a) $\frac{3}{-9}=\frac{9}{-27};\frac{-9}{3}=\frac{-27}{9};\frac{3}{9}=\frac{-9}{-27};\frac{9}{3}=\frac{-27}{-9}$
b)
$\frac{2}{4}=\frac{4}{8};\frac{2}{4}=\frac{8}{16};\frac{2}{4}=\frac{16}{32}$
$\frac{4}{2}=\frac{8}{4};\frac{4}{2}=\frac{16}{8};\frac{4}{2}=\frac{32}{16}$
$\frac{4}{8}=\frac{8}{16};\frac{4}{8}=\frac{16}{32}$
$\frac{8}{4}=\frac{16}{8};\frac{8}{4}=\frac{32}{16}$
$\frac{8}{2}=\frac{16}{4}=\frac{32}{8}$ và lất ngược lại nữa