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1.Set Theory
medium
If $aN = \{ ax:x \in N\} ,$ then the set $3N \cap 7N$ is .....$N$
A
$21$
B
$10$
C
$4$
D
None of these
Solution
(a) $3N = \{ x \in N:x$ is a multiple of $3\}$
$7N = \{ x \in N:x$ is a multiple of $7\}$
$\therefore 3N \cap 7N$$ = \{ x \in $ is a multiple of $3$ and $7\}$
$ = \{ x \in N:x$ is a multiple of $3$ and $7\}$
$ = \{ x \in N:x$ is a multiple of $21\}=21N.$
Standard 11
Mathematics
