$\begin{array}{l}\text{- Ta có : $\dfrac{2019}{2020}=\dfrac{2020-1}{2020}=1-\dfrac1{2020}\\\dfrac{2020}{2021}=\dfrac{2021-1}{2021}=1-\dfrac1{2021}$}\\\text{- Vì $2020<2021$}\\\to\dfrac1{2020}>\dfrac1{2021}\\\to1-\dfrac1{2020}<1-\dfrac1{2021}\\\to\dfrac{2019}{2020}<\dfrac{2020}{2021} \end{array}$
