a) $(\frac{-4}{9})+(\frac{5}{-9})=-1$
b) $(\frac{-13}{33})+(\frac{-14}{33}) =\frac{-27}{33}> (\frac{-10}{11})=\frac{-30}{33}$
c) $\frac{1}{5}+(\frac{-2}{3}) =\frac{-7}{15}$ và $ \frac{2}{5}+(\frac{-4}{5})=\frac{-2}{5}=\frac{-6}{15}$
Vậy $\frac{1}{5}+\frac{-2}{3}<\frac{2}{5}+(\frac{-4}{5})$