a) [tex]A\cap B=\varnothing\Rightarrow b<c[/tex] hoặc [tex]d\leq a[/tex]
b) [tex]A\cap B\neq \varnothing \Rightarrow [/tex] [tex]\left\{\begin{matrix}b\geq c & \\ c>a & \end{matrix}\right. \or \left\{\begin{matrix}d\geq a & \\ b\geq d & \end{matrix}\right.[/tex]