等差数列an ,前n项和为Sn,a1>0,且s12>0,s13

等差数列{an}中,a1>0,前n项和为Sn,且S9>0,S10 等差数列{an}中,前n项和为Sn,a1>0,S12>0,S13 等差数列an ,前n项和为Sn,a1>0,且s12>0,s13 等差数列｛an｝中,前n项和为sn,a1>0,且s12>0,s13 设等差数列{an}的前n项和为Sn,a1>0,S12>0,S13 等差数列{An}中,前n项和为Sn A1>0 且s12>0 s13 设等差数列{an}的前n项和为Sn 若a1=Sn> 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}前n项和为Sn,且a1>0,S13=S19,求Sn的最大值 已知数列{an}的前n项和为Sn,且满足an+2Sn*Sn-1=0,a1=1/2.求证：{1/Sn}是等差数列 已知数列an是等差数列,且a1不等于0,Sn为这个数列的前n项和,求limnan/Sn.limSn+Sn-1/Sn+Sn-1 等差数列an的前n项和为Sn,lim(Sn/n^2)=-a1/9＜0,求n为何值时.sn最大 已知等差数列an的前n项和为Sn,且a1^2+a8^2 已知等差数列an前n项和为sn,a1=-4,a8=-18 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｝前n项和为Sn,已知a1=13,S3=S11,n为什么时,Sn最大 设an公差不为0的等差数列.(1)前n项和为Sn,Sn=110,a1.a2.a4为等比数列.求an通项公式. 等差数列{an}中,已知a1>0,Sn为数列的前n项和,若S9>0,S10