Ta có: \(-\frac{72}{45}=\frac{-72:9}{45:9}=\frac{-8}{5}\)
\(\frac{9}{21}=\frac{9:3}{21:3}=\frac{3}{7}\)
\(-\frac{16}{44}=\frac{-16:4}{44:4}=-\frac{4}{11}\)
\(\frac{-15}{-35}=\frac{-15:\left(-5\right)}{-35:\left(-5\right)}=\frac{3}{7}\)
\(\frac{-20}{-55}=\frac{-20:\left(-5\right)}{-55:\left(-5\right)}=\frac{4}{11}\)
\(-\frac{32}{20}=\frac{-32:4}{20:4}=-\frac{8}{5}\)