若{an}是个等差数列,前n项和为sn,已知S4=6 S8=20 求S12

设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列 若{an}是个等差数列,前n项和为sn,已知S4=6 S8=20 求S12 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 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 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. an是等差数列,sn为前N项和,Sm/Sn=m2/n2 则am/an= 设数列｛an｝的前n项和为sn,若对于所有的正整数n,都有sn=n（a1+an）/2,证明｛an｝是等差数列设数列｛an｝的前n项和为sn,若对于所有的正整数n,都有sn=n（a1+an）/2,证明｛an｝是等差数列 an为等差数列,Sn是前n项和,若Sm=n,Sn=m,求证Sm+n=-(m+n) 已知数列{an}的前n项和为Sn=n^2-3n,求证：数列{an}是等差数列