Tính nhanh: c) [ ( − 5/7 ) + 3/5 ] : 2020/2021 + ( 2/5 − 2/7 ) : 2020/2021

c) $[(-\frac{5}{7})+\frac{3}{5}]:\frac{2020}{2021}+(\frac{2}{5}-\frac{2}{7}):\frac{2020}{2021}$

$=[(-\frac{5}{7})+\frac{3}{5}].\frac{2021}{2020}+(\frac{2}{5}-\frac{2}{7}).\frac{2021}{2020}$

$=[(-\frac{5}{7})+\frac{3}{5}+\frac{2}{5}-\frac{2}{7}].\frac{2021}{2020}$

$=(1-1).\frac{2021}{2020}=0.\frac{2021}{2020}=0$