Using that for any sets $\mathrm{A}$ and $\mathrm{B},$
$A \cup(A \cap B)=A$
To show: $A \cup(A \cap B)=A$
We know that
$A \subset A$
$A \cap B \subset A$
$\therefore A \cup(A \cap B) \subset A$ ..........$(1)$
Also, $A \subset A \cup(A \cap B)$ ..............$(2)$
$\therefore$ From $(1)$ and $(2), A \cup(A \cap B)=A$
If $A=\{3,6,9,12,15,18,21\}, B=\{4,8,12,16,20\},$ $C=\{2,4,6,8,10,12,14,16\}, D=\{5,10,15,20\} ;$ find
$A-C$
If $X=\{a, b, c, d\}$ and $Y=\{f, b, d, g\},$ find
$X \cap Y$
If $X=\{a, b, c, d\}$ and $Y=\{f, b, d, g\},$ find
$X-Y$
Find the union of each of the following pairs of sets :
$A = \{ x:x$ is a natural number and $1\, < \,x\, \le \,6\} $
$B = \{ x:x$ is a natural number and $6\, < \,x\, < \,10\} $
$A$ and $B$ are two subsets of set $S$ = $\{1,2,3,4\}$ such that $A\ \cup \ B$ = $S$ , then number of ordered pair of $(A, B)$ is