Đáp án:
$\begin{array}{l}
A = \frac{{{{2019}^{2019}} + 1}}{{{{2019}^{2020}} + 1}};B = \frac{{{{2019}^{2020}} - 2}}{{{{2019}^{2021}} - 2}}\\
Do:B = \frac{{{{2019}^{2020}} - 2}}{{{{2019}^{2021}} - 2}} < 1\\
\Rightarrow \frac{{{{2019}^{2020}} - 2}}{{{{2019}^{2021}} - 2}} < \frac{{{{2019}^{2020}} - 2 + 2021}}{{{{2019}^{2021}} - 2 + 2021}}\\
\Rightarrow B < \frac{{{{2019}^{2020}} + 2019}}{{{{2019}^{2021}} + 2019}} = \frac{{2019\left( {{{2019}^{2019}} + 1} \right)}}{{2019\left( {{{2019}^{2020}} + 1} \right)}} = \frac{{{{2019}^{2019}} + 1}}{{{{2019}^{2020}} + 1}}\\
Vậy\,B < A
\end{array}$
