Đáp án: $\left\{ \begin{array}{l}
x = y\,khi:a = \dfrac{{2020}}{{2019}}\\
x > y\,khi:a > \dfrac{{2020}}{{2019}}\\
x < y\,khi:a < \dfrac{{2020}}{{2019}}
\end{array} \right.$
Giải thích các bước giải:
$\begin{array}{l}
+ Khi:x = y\\
\Leftrightarrow 2020.a = 2020 + a\\
\Leftrightarrow 2019.a = 2020\\
\Leftrightarrow a = \dfrac{{2020}}{{2019}}\\
+ Khi:x > y\\
\Leftrightarrow 2020a > 2020 + a\\
\Leftrightarrow 2019a > 2020\\
\Leftrightarrow a > \dfrac{{2020}}{{2019}}\\
+ Khi:x < y\\
\Leftrightarrow a < \dfrac{{2020}}{{2019}}\\
Vay\,\left\{ \begin{array}{l}
x = y\,khi:a = \dfrac{{2020}}{{2019}}\\
x > y\,khi:a > \dfrac{{2020}}{{2019}}\\
x < y\,khi:a < \dfrac{{2020}}{{2019}}
\end{array} \right.
\end{array}$