以知等差数列an前N项和为Sn=-2n^2-n,求通项an的表达式为什么S(n-1)=-2(n-1)²-(n-1)=-2n²+3n-1呢？

等差数列{an}前n项和为Sn=3n-2n^2,求an 等差数列{an}前n项和为sn,求证S2n-1=(2n-1)an 已知等差数列an中,前n项和sn=n^2-15n,则使sn为最小值的n 以知等差数列an前N项和为Sn=-2n^2-n,求通项an的表达式为什么S(n-1)=-2(n-1)²-(n-1)=-2n²+3n-1呢？ 等差数列前n项和为Sn=1/2(An+1/An),求通项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｝前n项和为Sn,且Sn=3n^2+n 求公差d 已知sn=32n-n^2求等差数列|an|的前n项和sn 等差数列前n项和为Sn,求证:S2n-1=(2n-1)an 等差数列{an} 前n项和为sn,则lim（n趋向无穷）【sn/（an^2）】=? 数列an ,a1=1,前n项和为Sn ,正整数n对应的n an Sn 成等差数列.1.证明{Sn+n+2}成等比数列,2.求{n+2/n(n+1)(1+an)}前n项和 设等差数列(An)的前n项和为Sn,以知A4=14.S10=185,求等差数列(An)的通项An 等差数列{an}前n项和为Sn,求证S 2n-1项=（2n-1）an 等差数列an前n项和为Sn,求证S（2n-1）=（2n-1）an 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 已知数列{An}的前n项和Sn=3n²-2n,证明数列{An}为等差数列 已知数列{an}的前N项和sn=n^2+n+1,an是否为等差数列? 已知数列{an}的前n项和Sn=-n^2+18n,求证:{an}为等差数列