Question

Is * defined on the set {1,2,3,4,5} by a*b=L.C.M of a and b ,a binary operation?

Answer 1

$a\ast b=$LCM of a and b

For $a=1;b=2;a\ast b=2ϵ\{1,2,3,4,5\}$

But for $a=2;b=3,a\ast b=6\notin \{1,2,3,4,5\}$

$\therefore \ast$ is not binary operation.