a) Đk: \(\hept{\begin{cases}x-m-1\ge0\\4x-m\ge0\end{cases}}\Leftrightarrow\hept{\begin{cases}m+1\le x\\\frac{m}{4}\le x\\x\ge0\end{cases}}\)
\(\Leftrightarrow\hept{\begin{cases}m+1\le0\\m\le0\end{cases}}\Leftrightarrow\hept{\begin{cases}m\le-1\\m\le0\end{cases}}\)
\(\Leftrightarrow m\le-1\)