$\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{2020.2021}$
$=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{2020}-\dfrac{1}{2021}$
$=\dfrac{1}{1}-\dfrac{1}{2021}$
$=\dfrac{2021-1}{2021}$
$=\dfrac{2020}{2021}$
Chúc bạn học tốt !!!