N!=1*2*3*****n (n+1)! / (n+3)!=(n+1)!/(n+1)!(n+2)(n+3)=1/(n+2)(n+3)
(n+1)! / (n+3)! = (n+1)! / (n+1)! (n+2)(n+3) = 1/ (n+2)(n+3)
спасибо