Đáp án:
$\begin{array}{l}
1)Dkxd:6 - 2x \ge 0\\
\Rightarrow 2x \le 6\\
\Rightarrow x \le 3\\
2)Khi:y = 2\\
\Rightarrow 2 = 2x - 1\\
\Rightarrow 2x = 3\\
\Rightarrow x = \dfrac{3}{2}\\
\Rightarrow \left( {\dfrac{3}{2};2} \right) \in y = m{x^2}\\
\Rightarrow 2 = m.{\left( {\dfrac{3}{2}} \right)^2}\\
\Rightarrow 2 = m.\dfrac{9}{4}\\
\Rightarrow m = \dfrac{8}{9}
\end{array}$
3) Hàm đồng biến khi x<-2020<0
$\begin{array}{l}
\Rightarrow \dfrac{{2 - a}}{{2 + a}} < 0\\
\Rightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
2 - a < 0\\
2 + a > 0
\end{array} \right.\\
\left\{ \begin{array}{l}
2 - a > 0\\
2 + a < 0
\end{array} \right.
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
a > 2\\
a > - 2
\end{array} \right.\\
\left\{ \begin{array}{l}
a < 2\\
a < - 2
\end{array} \right.
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
a > 2\\
a < - 2
\end{array} \right.
\end{array}$
Vậy a>2 hoặc a<-2 thì hàm đồng biến khi x<-2020
