{an}是等差数列,前n项和为Sn,若a3=18-a4,求S7

设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列 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=? 等差数列前n项和为Sn,若a（n+1）=3Sn,a1=1,则通项an? {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 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? 已知an是等差数列,前n项和为Sn,求证:S3n=3(S2n-Sn) 设Sn为等差数列an的前n项和.求证Sn/n为等差数列 (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 {an}是等差数列前n项和Sn已知Sm=a Sn-Sn-m=b 求Sn 等差数列{an}的前n项和为sn,a10 设等差数列{an}的前n项和为Sn 若a1=Sn> 设等差数列{an}公差d是2,前n项和为Sn,则lim(an^2-n^2)/Sn 设等差数列{an}公差d是2,前n项和为Sn,则lim(an^2-n^2)/Sn 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. 等差数列〔an 〕的前n 项和为Sn,已知Sm等于a ,Sn 减Sn-m 等于b ,n ,m 是正的等差数列〔an 〕的前n 项和为Sn，已知Sm等于a Sn 减Sn-m 等于b n m 是正的自然数。n 大于m 求Sn 在等差数列{an}中,其前n项和为Sn,若Sn=an2+(a+1)n+a+2,则an= 在等差数列{an}中,其前n项和为Sn,若Sn=an2+(a+1)n+a+2,则an=