等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=

已知数列{an}中,a2=2,前n项和为Sn,且Sn=n(an+1)/2证明数列{an+1-an}是等差数列 设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列 已知数列{an}中,an>0,前n项和为Sn,且满足Sn=1/8(an+2)^2.求证数列{an}是等差数列. 等差数列{an}中,Sn是前n项和,a3+a8>9,S9 已知等差数列{an}中,Sn是它的前n项和,若S16>0,S17 在等差数列{an}中,Sn是前n项和,a8=12,则S15= 已知等差数列{an}中,Sn是它的前n项和,若S16>0,S17 等差数列(An)中,若3a5=8a12>0,Sn是等差数列(An)前n项和,则Sn取最大值时,n等于多少 等差数列{an}中,Sn是{an}前n项的和,S6=9,S9=6,则S15=? 等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15= 等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15= 已知数列{an}的前n项和sn满足sn=an^2+bn,求证{an}是等差数列 设Sn为等差数列{an}的前n项和,已知a6+a70,那么Sn中最小的是 设Sn为等差数列{an}的前N项和,已知 a6 +a70,那么Sn 中最小的是 (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 在等差数列{|an|}中,an=-2n+10,求数列{|an|}的前n项和Sn 等差数列{an}中,a1>0,3a4=7a7,sn是数列{an}的前n项和,则sn取得最大值是n= 等差数列an中,a50,且a6>/a5/,sn为数列an的前n项和,则使sn>0的n的最小值是