等差数列{an}的前n项和为Sn,且满足Sn=S(19-n),(n属于自然数且小于19）,当绝对值an最小时候,n=多少?感激不尽

设等差数列{an}的前n项和为Sn,且满足S15>0,S16 已知数列{an}的前n项和为Sn,且满足an+2Sn*Sn-1=0,a1=1/2.求证：{1/Sn}是等差数列 已知等差数列an的前n项和为sn 且满足Sn=n²+n,则通项公式an=? 已知等差数列an的前n项和为Sn,且满足a2+a4=14,S7=70 求证等差数列!已知数列an的各项均为正数,前n项和为Sn,且满足2Sn=a∧2n+n-4 (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 已知数列｛an｝的前n项和为sn,且满足sn＝n 已知等差数列{an}的前n项和为Sn,且满足S2=6,a3+a4=22,求an与Sn 设数列An的前n项和为Sn,满足2Sn=An+1 -2^n+1+1,且A1.A2+5.A3成等差数列 求数列的设数列An的前n项和为Sn,满足2Sn=An+1 -2^n+1+1,且A1.A2+5.A3成等差数列 求数列的通项公式 已知数列an,a1=1,他的前n项和为Sn,且满足an+1=Sn+n+1.求证：（1）an+1是等差数列 （2）求an和Sn的表达式 设等差数列{an}满足3a8=5a13,且a1>0,Sn为其前n项和,则Sn中最大的是前几项和? 数列（an}的前n项和为Sn,且满足an=-3Sn*Sn-1(n>=2),a1=1/3 1.证（1/Sn}是等差数列数列（an}的前n项和为Sn,且满足an=-3Sn*Sn-1(n>=2),a1=1/3 1.证（1/Sn}是等差数列 2.令bn=2(1-n)an(n>=2)求证b2平方+b3平方+.+bn平方 在递减的等差数列an中,前n项和为Sn,且满足a19*a21=1260,a14+a16+a18+a22+a24+a26=216,在满足Sn>an的所有n中取最大整数N,求Sn 设Sn为等差数列an的前n项和.求证Sn/n为等差数列 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. 已知数列{an}的前n项和为sn,且满足a1=1/2,an+2snsn-1=0(n≥2)(1)问:数列{1/Sn}是否为等差数列?(2)求Sn和an 高三数列难题已知正项等差数列An>0,且前n项和为Sn,满足{An}和{根号Sn}都是等差数列,且公差相等,求An的通项公式, 已知数列an的前n项和为Sn,且满足Sn=Sn-1/2Sn-1+1,a1=1/2(1)求证:1/Sn是等差数列(2（2）求an的表达式