数列题,数列题,
数列题,a1+2a2+...+nan=n(n+1)(n+2)a1+2a2+...+(n-1)a(n-1)=(n-1)n(n+1)两式相减,得nan=3n(n+1)即an=3n+3所以Sn=(6+3n+3)n/2=3/2n^2+9/2n