a;\(aaa=111\cdot a\)
\(\Rightarrow aaa=3\cdot37\cdot a\)
\(\Rightarrow aaa⋮37\left(3a\inℕ\right)\)
b;\(a\ge b;ab-ba=10a+b-10b-a\)
\(\Rightarrow ab-ba=9a-9b\)
\(\Rightarrow ab-ba=9\left(a-b\right)\)
\(\Rightarrow ab-ba⋮9\left(a\ge b\Rightarrow a-b\ge0\right)\)
\(n\left(n+1\right)\)
*;\(n⋮2\Rightarrow n\left(n+1\right)⋮2\)
*;\(n⋮̸2\Rightarrow n+1⋮2\Rightarrow n\left(n+1\right)⋮2\)