Commutative Laws of Sets
 Theorem 1 (Commutative Law for the Union of Two Sets): If $A$ and $B$ are sets then $A \cup B = B \cup A$.
• Proof: Suppose that $x \in A \cup B$. Then $x \in A$ or $x \in B$ or $x \in A \cap B$ to which we write that $x \in B \cup A$. Therefore $A \cup B = B \cup A$. $\blacksquare$
 Theorem 2 (Commutative Law for the Intersection of Two Sets): If $A$ and $B$ are sets then $A \cap B = B \cap A$.
• Proof: Suppose that $x \in A \cap B$. If $x$ is both in $A$ and $B$, then we can also say that $x$ is in both $B$ and $A$ or rather $x \in B \cap A$. Therefore $A \cap B = B \cap A$. $\blacksquare$