Q.If set $A = \{11, 12, 13\}$ and set $B = \{12, 13, 14\}$, then the number of elements in set $A \cup B$ is :

$A\triangle B=(A-B)\cup(B-A)$ — elements in exactly one of the two sets.

The symmetric difference of two sets $A$ and $B$ (denoted $A\triangle B$ or $A\ominus B$) is the set of all elements that belong to either $A$ or $B$ but not to both:

$A\triangle B=(A-B)\cup(B-A)=(A\cup B)-(A\cap B)$

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$A\cup B=\{11,12,13,14\}$, so $n(A\cup B)=4$.

List the union directly:

$A\cup B=\{11,12,13\}\cup\{12,13,14\}=\{11,12,13,14\}.$

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