Let $A = \{a, e, i, o, u\}$ and $B = \{a, i, u\}$. Show that $A \cup B = A$.

(N/A) We have $A = \{a, e, i, o, u\}$ and $B = \{a, i, u\}$.
$A \cup B = \{a, e, i, o, u\} \cup \{a, i, u\} = \{a, e, i, o, u\}$.
Since the resulting set is exactly $A$,we have $A \cup B = A$.
This example illustrates that the union of a set $A$ and its subset $B$ is the set $A$ itself,i.e.,if $B \subset A$,then $A \cup B = A$.