an是等差数列,Sn是前n项和,若OB=a2 OA+a2009 OC,A.B.C三点共线(直线不过原点),求S2010.

an是等差数列,Sn是前n项和,若OB=a2 OA+a2009 OC,A.B.C三点共线(直线不过原点),求S2010. 设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列 已知an是等差数列,前n项和为Sn,求证:S3n=3(S2n-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=? {an}是等差数列,前n项和为Sn,若a3=18-a4,求S7 设｛an｝是等差数列前n项和为Sn,若S4>=10,S5 {an}是等差数列前n项和Sn已知Sm=a Sn-Sn-m=b 求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? (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 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 已知Sn是等差数列{An}的前n项和,S10>0并且S11=0,若Sn 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 an是等差数列,前n项和是Sn,若limSn/(an)^2=1/6,求公差d. 等差数列｛an｝3a4=7a7,且a1>0sn是等差数列｛an｝的前n项和,若sn取最大值, 设数列{an}的前n项和为Sn,若对任意正整数,都有Sn=n(a1+an)/2,证明{an}是等差数列. 已知等差数列{an}的前n项和是sn=32n一n*n,求{|an|}的前n项和sn Sn是等差数列an的前n项和,若S1=1,S2=4,则an= an是等差数列,sn为前N项和,Sm/Sn=m2/n2 则am/an=