等差数列{an}j的前n项和为Sn,若a3+a17=10则S19=

等差数列{an}j的前n项和为Sn,若a3+a17=10则S19= 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> 等差数列前n项和为Sn,若a（n+1）=3Sn,a1=1,则通项an? 等差数列an的前n项和为Sn,已知Sm=a,Sn-Sn-m=b,m、n属于自然数且n>m,求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=? 设数列an的前n项和为Sn,满足an+sn=An^2+Bn+1(A不等于0)an为等差数列,求(B-1)/A 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? 设等差数列an的前n项和为Sn,若S4>=10,S5 设等差数列{AN}的前N项和为SN,若S9>0,S10 设等差数列 {an}的前n 项和为Sn,若S9>0 ,S10 已知等差数列{an} 的前n项和为Sn,若S12>0,S13 在等差数列{an},设Sn为它的前n项和,若S15>0,S16 等差数列｛an｝的前n项和为sn,若S15=90,则a8= 设等差数列{an}的前n项和为Sn,若-a2013 若等差数列an的前n项和为sn 则三点,,共线