等差数列前n项和为Sn=(a+1)n^2+a 某三角形三边之比为a2:a3:a4则该三角形最大角为

等差数列{An}中,前n项和Sn=an2+（a-1).n+(a+2),则An=为什不能用Sn-Sn-1算 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,求证S2n-1=(2n-1)an 等差数列前n项和为Sn,若a（n+1）=3Sn,a1=1,则通项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 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 等差数列{an}前n项和为Sn=3n-2n^2,求an 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 等差数列{a(n)}中,a1=20,前n项和为Sn,且S10=S15.（1）求前n项和Sn.（2）当n为何值时,Sn有最大值. 等差数列{an}=2n+1,前n项和为Sn,求1/S1+1/S2+.+1/Sn 已知等差数列{an}的前n项和为Sn,如果Sn=(an+1/2)^2(n∈N+0,bn=(-1)^n*Sn已知等差数列{an}的前n项和Sn,如果Sn=(an+1/2)^2(n∈N+0,bn=(-1)^n*Sn,试求{bn}的前n项和Tn 两个等差数列{an},(bn}前n项和分别为Sn,S'n,若Sn/S'n=(2n+3)/(3n-1)求a9/b9 等差数列an的前n项和为Sn=n^2+4n-1,则通项公式为? 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= 已经等差数列的前n项和为Sn=n^2+a,求a值已经等差数列的前n项和为Sn=n^2+a,求a值 在等差数列中,前n项和Sn=a,前2n项和S2n=b,则前3n项和S3n为(过程) 等差数列{an}前n项和为Sn,求证S 2n-1项=（2n-1）an