[tex]\it A^2_n < 30 \Rightarrow \dfrac{n!}{(n-2)!} < 30 \Rightarrow \dfrac{(n-2)!\cdot(n-1)\cdot n}{(n-2)!} < 30 \Rightarrow (n-1)n < 30 \Rightarrow \\ \\ \Rightarrow n\in\{2,\ \ 3,\ \ 4,\ \ 5\}[/tex]
[tex]\bf Observa\c{\it t}ie:\\ \\ \it A^2_n \Rightarrow n\geq2[/tex]
