Nếu bạn muốn hỏi ý này:
$\frac{1}{1.3}$ + $\frac{1}{3.5}$ + ...+ $\frac{1}{2019.2021}$
$=\frac{1}{2}(\frac{2}{1.3}$ + $\frac{2}{3.5}$ + ...+ $\frac{2}{2019.2021})$
$=\frac{1}{2}(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+....+\frac{1}{2019}-\frac{1}{2021})$
$=\frac{1}{2}(1-\frac{1}{2021})$
$=\frac{1}{2}.\frac{2020}{2021}$
$=\frac{1010}{2021}$