∆ Определение 5.

The union of sets $A$ and $B$ is a set consisting of all elements $x$ such that $x \in A$ or $x \in B$. Notation: $A \cup B$. The intersection of sets $A$ and $B$ is a set consisting of all elements $x$ such that $x \in A$ and $x \in B$. Notation: $A \cap B$. The difference of sets $A$ and $B$ is a set consisting of all elements $x$ such that $x \in A$ and $x \notin B$. Notation: $A \setminus B$.