数列{an}的通项为an=1/2n(2n+2),求{an}的前n项和
问题描述：
数列{an}的通项为an=1/2n(2n+2),求{an}的前n项和
答
an=1/[2n(2n+2)]
= (1/4) [ 1/n - 1/(n+1) ]
Sn =a1+a2+...+an
=(1/4) [1-1/(n+1) ]
= n/[4(n+1)]