已知數列{an}的通項公式an=n²;+n,則{an}的前n項和Sn=?

已知數列{an}的通項公式an=n²;+n,則{an}的前n項和Sn=?

an =n^2+n
= n(n+1)
=(1/3)[n(n+1)(n+2)-(n-1)n(n+1)]
Sn =(1/3){ summation(i:1->n)[i(i+1)(i+2)-(i-1)i(i+1)] }
=(1/3)n(n+1)(n+2)