20.
\(2x+1\le5\Leftrightarrow2x\le4\)
\(\Leftrightarrow x\le2\)
C đúng
21.
\(B\subset A\Leftrightarrow\left\{{}\begin{matrix}m-7\ge-4\\m\le3\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}m\ge3\\m\le3\end{matrix}\right.\) \(\Rightarrow m=3\)
C đúng
22.
\(A\subset B\Leftrightarrow\left\{{}\begin{matrix}m-1< 4\\-2< 2m+2\\m-1\ge-2\\2m+2\ge4\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}m< 5\\m>-2\\m\ge-1\\m\ge1\end{matrix}\right.\)
\(\Rightarrow1\le m< 5\)
A đúng
23.
\(\Leftrightarrow\left\{{}\begin{matrix}m\ge1\\m+1\le3\end{matrix}\right.\) \(\Rightarrow1\le m\le2\)
C đúng
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