Question

If A and B are disjoint, then $n(A\cup B)$ = ___.

• A. n(A)
• B. n(B)
• C. n(A)+n(B)
• D. n(A).n(B)

Answer 1

The correct option is C n(A)+n(B)

Since A and B are disjoint, we have $A\cap B=\varphi =\left\{\right\}.$
$\Rightarrow n(A\cap B)=0$
$n(A\cup B)=n\left(A\right)+n\left(B\right)-n(A\cap B)$
$\Rightarrow n(A\cup B)=n\left(A\right)+n\left(B\right)-0=n\left(A\right)+n\left(B\right)$