Question:
For any two sets $A$ and $B$, prove that: $A \cap B=\phi \Rightarrow A \subseteq B$.
Solution:
Let $a \in A \Rightarrow a \notin B \quad(\because A \cap B=\phi)$.
$\Rightarrow a \in B$
Thus, $a \in A$ and $a \in B^{\prime} \Rightarrow A \subseteq B$ '.