an=1/n(n+1)=[1/n-1/(n+1)]Sn=a1+a2+...+an =[1-1/2+1/2-1/3+...+1/n-1/(n+1)] =[1-1/(n+1)] =n/(n+1)
Sn=n+〔n!+(n-1)!+…+3!+2!+1!〕/n!
