求下面数列的前n项和:
1,3,5,7,…
数列1,2,3,4,…的前n项和是__________.
数列{an}的前n项和为Sn,若an=,则S4=________.
(1+2n)=________.
已知数列{an}中,a1=1,(n+1)an+1=nan(n∈N*),则该数列的通项公式an=________.
在数列{an}中,若a1=1,an+1=an+2(n≥1),则该数列的通项an=________.