Question

$\cap$If A and B are two disjoint sets, then $n\left(A\cup B\right)$ is equal to
(a) $n\left(A\right)+n\left(B\right)$
(b) $n\left(A\right)+n\left(B\right)-n\left(A\cap B\right)$
(c) $n\left(A\right)+n\left(B\right)+n\left(A\cap B\right)$
(d) $n\left(A\right)n\left(B\right)$
(e) $n\left(A\right)-n\left(B\right)$

Answer 1

(a) $n\left(A\right)+n\left(B\right)$

Two sets are disjoint if they do not have a common element in them, i.e., A$\cap$B = $\varnothing$.
∴ $n\left(\mathrm{A}\cup \mathrm{B}\right)=n\left(\mathrm{A}\right)+n\left(\mathrm{B}\right)$