Question:
$\cap$ If $A$ and $B$ are two disjoint sets, then $n(A \cup B)$ is equal to
(a) $n(A)+n(B)$
(b) $n(A)+n(B)-n(A \cap B)$
(c) $n(A)+n(B)+n(A \cap B)$
(d) $n(A) n(B)$
(e) $n(A)-n(B)$
Solution:
(a) $n(A)+n(B)$
Two sets are disjoint if they do not have a common element in them, i.e., $A \cap B=\emptyset$.
$\therefore n(\mathrm{~A} \cup \mathrm{B})=n(\mathrm{~A})+n(\mathrm{~B})$