等差数列{An}的前n项和为Sn=312,它的前四项的和为32,后四项的和为176,若{An}只有n项,则项数n等于?前四项和后四项的和是四倍的An+1 ,是怎么来的。还是不懂。

设Sn为等差数列an的前n项和.求证Sn/n为等差数列 等差数列{an}的前n项和为sn,a10 设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列 设等差数列{an}的前n项和为Sn 若a1=Sn> (1)已知数列an的前n项和为sn满足sn=an²+bn,求证an是等差数列(2)已知等差数列an的前n项和为sn,求证数列sn/n也成等差数列 已知等差数列an中,前n项和sn=n^2-15n,则使sn为最小值的n 等差数列an的前n项和为Sn,已知a5=11 a8=5求an和Sn 等差数列{an}前n项和为Sn=3n-2n^2,求an 已知sn=32n-n^2求等差数列|an|的前n项和sn 记数列{an}的前n项和为Sn,若{Sn/an}是公差为d的等差数列,则{an}为等差数列的充要条件是d=? 等差数列{an}前n项和Sn=-n²+n,求等差数列的首项 已知公差不为0的等差数列{An}的首项A1=1,前n项和为Sn,若数列{Sn/An}是等差数列,求An? 等差数列{An}的前n项和为Sn,S10=45,a4+a6= 已知Sn为等差数列an的前n项和,a6=100,则S11= 等差数列｛an｝的前n项和为sn,若S15=90,则a8= 已知Sn为等差数列｛an｝的前n项和,S11=55,则a6= 等差数列{an}的前n项和为Sn,且S10-S5=3,S15=? 设等差数列an的前n项和为Sn,若S4>=10,S5