n!=1×2×3×..n, (n+2)!=1×2×3×..n×(n+1)×(n+2) deci(n+2)!/n!=(n+1)(n+2)=12deci n+1=3 si n+2=4, n=2
= (n+1)(n+2) = 12, n∈N ⇒ n=2.