等差数列｛an｝的前n项和为sn,若S15=90,则a8=

等比数列{an},前n项和为Sn,若S1,2S2,3S3成等差数列,求{an}的公比 等比数列{an}的前n项和为Sn,已知S1,S3,S2成等差数列1 ,求｛an｝的公比q2 ,若a1-a3=3求Sn 等比数列an的前n项和为sn 已知s1 s3 s2成等差数列 求an的公比q 求若a1-a3=3 求sn 等比数列{an}的前n项和为Sn.S1.2S2.3S3诚等差数列,则{An}的公比 设等差数列an前n项和为Sn,S2010-S1=1,S2011=? 等差数列题!记等差数列｛An}的前n项和为Sn,若S1=4,S4=20,则该数列的公差d=?3Q! Sn是等差数列an的前n项和,若S1=1,S2=4,则an= 已知等差数列an的前n项和为Sn,若S3/3-S1=1则数列an的公差为? 已知等比数列{an}的公比为q,前n项和为Sn,若S2011,S2010,S2012成等差数列,且S1=1,则an等于 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}=2n+1,前n项和为Sn,求1/S1+1/S2+.+1/Sn 数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn 设等差数列{an}的前n项和为Sn,a1=3/2,且S1,S2,S4成等差数列,求Sn 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 等比数列{an}中,前n项和为sn,已知S1,S3,S2成等差数列,求{an}的公比Q.已知a1-a3=3,求sn? 等比数列{an}中,前n项和为sn,已知S1,S3,S2成等差数列,求{an}的公比Q.已知a1-a3=3,求sn?RT. 设等差数列{an}的前n项和为Sn 若a1=Sn>