数列{an}是首项为2,公差为1的等差数列,其前n项和为Sn,求数列an的通项公式an及前n项和Sn
问题描述：
数列{an}是首项为2,公差为1的等差数列,其前n项和为Sn,求数列an的通项公式an及前n项和Sn
答
an=a1+(n-1)d
=2+(n-1)*1=n+1
Sn=a1n+n(n-1)*d/2
=2n+n(n-1)/2=2n+n^2/2-n/2=(n^2+3n)/2