\(A=\sqrt{x-2019}+\sqrt{2020-x}\ge\sqrt{x-2019+2020-x}=1\)
\(A_{min}=1\) khi \(\left[{}\begin{matrix}x=2019\\x=2020\end{matrix}\right.\)
\(A\le\sqrt{2\left(x-2019+2020-x\right)}=\sqrt{2}\)
\(A_{max}=\sqrt{2}\) khi \(x-2019=2020-x\Leftrightarrow x=\frac{4039}{2}\)