a1+2a2+3a3+…+nan=n(n+1)(n+2), (1)
a1+2a2+3a3+…+(n-1)an-1=(n-1)(n+1)n (2)
(1)-(2)得:nan=n(n+1)(n+2-n+1)
an=3(n+1)
Sn=3n[2+n+1]/2=(3n^2+9n)/2
a1+2a2+3a3+…+nan=n(n+1)(n+2), (1)
a1+2a2+3a3+…+(n-1)an-1=(n-1)(n+1)n (2)
(1)-(2)得:nan=n(n+1)(n+2-n+1)
an=3(n+1)
Sn=3n[2+n+1]/2=(3n^2+9n)/2