设Sn是等差数列{an}的前n项和,已知S8=108,Sn=630,S(n-8)=234,求n.

来源：学生作业帮助网编辑：作业帮时间：2024/04/28 04:23:49

设Sn是等差数列{an}的前n项和,已知S8=108,Sn=630,S(n-8)=234,求n.
设Sn是等差数列{an}的前n项和,已知S8=108,Sn=630,S(n-8)=234,求n.

设Sn是等差数列{an}的前n项和,已知S8=108,Sn=630,S(n-8)=234,求n.
a(n-7)+a(n-6)+a(n-5)+······+an=Sn-S(n-8)
=630-234
=396 (1)
a8 + a7 + a6 +……+ a1 = S8 =108 （2）
1式加2式可得：
[a(n-7)+a8] + [a(n-6)+a7] + ······+（an+a1) =396+108
8(an+a1) =504
an+a1 = 63 (根据若m+n=p+q,则 am+an = ap+aq)
Sn=(a1+an)n/2
630=63n/2
n=20

设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列设Sn为等差数列{an}的前n项和,已知a6+a70,那么Sn中最小的是设Sn为等差数列{an}的前N项和,已知 a6 +a70,那么Sn 中最小的是设Sn是等差数列{an}的前n项和,已知S6=36,Sn=324,Sn-6=144（n>6),则n等于? 设Sn为等差数列an的前n项和.求证Sn/n为等差数列设等差数列{an}的前n项和为Sn,已知a3=11,S15*S16 设等差数列{an}的前n项和为Sn,已知S12>0,S13 设等差数列{an}的前n项和sn,已知a3=12,s12>0.s13 设Sn是等差数列an的前n项和,已知a2=3,S11=121,则S7等于已知等差数列{an}中Sn是它的前n项和,设a6=2,S10=10 设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于? 设Sn是等差数列 an 的前n 项和,已知a2=3,a6=11,求S7. 设Sn是等差数列{an}的前n项和,已知a2=3,a6=11,则S7=? 设Sn是等差数列｛an｝的前n项和,已知a2=3,a6=11,则S7等于? 设Sn是等差数列(an)的前n项和,已知a2等于3,a6等于11,则S7等于设Sn是等差数列｛an｝的前n项和,已知a2=3,S11=121,则S7= (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也成等差数列设Sn是等差数列{an}的前n项和,已知S8=108,Sn=630,S(n-8)=234,求n.