Thay dấu ? bằng dấu >, <, = thích hợp.

c) Ta có: $\frac{1}{5}+\left(\frac{-2}{3}\right)=\frac{3}{15}+\left(\frac{-10}{15}\right)=\frac{-7}{15}$.

$\frac{2}{5}+\left(\frac{-4}{5}\right)=\frac{-2}{5}=\frac{-6}{15}$

Vì −7 < −6 nên $\frac{-7}{15}<\frac{-6}{15}$ .

Vậy $\frac{1}{5}+\left(\frac{-2}{3}\right)\boxed{<}\frac{2}{5}+\left(\frac{-4}{5}\right)$ .