4. a, M có nghĩa khi [tex](x-1)(x+3)\geq 0\Leftrightarrow x\geq 1 or x\leq -3[/tex]
N có nghĩa khi [tex]\left\{\begin{matrix} x-1\geq 0 & & \\ x+3\geq 0 & & \end{matrix}\right. \Leftrightarrow \left\{\begin{matrix} x\geq 1 & & \\ x\geq -3 & & \end{matrix}\right.[/tex]
b, M=N khi \begin{matrix} x\geq 1 & & \\ & & \end{matrix}