(n+3)! / (n+1)!
[ n + 3] ! / [ n + 1] ! =
[ (n + 3) (n + 2) (n + 1) * .......* 1] / [ (n + 1) * .......* 1 ] =
[ ( n + 3) (n + 2) ] =
n^2 + 5n + 6
(n+3)! / (n+1)! = (n+2) (n+3)
