等差数列{an}中,前n项和为Sn,若S3=21,S8=136,则Sn=?

来源：学生作业帮助网编辑：作业帮时间：2024/05/11 01:06:57

等差数列{an}中,前n项和为Sn,若S3=21,S8=136,则Sn=?
等差数列{an}中,前n项和为Sn,若S3=21,S8=136,则Sn=?

等差数列{an}中,前n项和为Sn,若S3=21,S8=136,则Sn=?
直接来s3=3*a1+3d=21
s8=8*a1+8*7/2*d=8*a1+28d=136
解方程得d=4
a1=3
sn=a1*n+n*(n-1)/2*d
=2n^2+n

S3=21
a1+a2+a3=21
a2=7
S8=136
S8=8(a1+a8)/2=4(a2+a7)=4(7+a7)=136
a7=27
d=(a7-a2)/5=(27-7)/5=4
a1=a2-d=7-4=3,an=a1+(n-1)d=3+4(n-1)=4n-1
所以Sn=n(a1+an)/2=n(3+4n-1)/2=n(2n+1)

在等差数列{an}中,其前n项和为Sn,若S10>0,S11 等差数列中前n项和为Sn若Sm等于n,Sn等于m求S{m+n} 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 已知等差数列an中,a1=1,前n项和Sn,若S(n+1)/Sn=(4n+2)/(n+1),求an 已知等差数列an中,a1=1,前n项和Sn,若S(n+1)/Sn=(4n+2)/(n+1),求an 等差数列{an}中a₁=1/3,前n项和为sn,若S₁,2S₂,3S₃成等比数列,则log3an= 等差数列{an}中,前n项和为Sn,若S3=21,S8=136,则Sn=? 设等比数列an的公比为q,前n项和为sn,若s(n+1),sn,s(n+2)成等差数列,求q的值设等比数列｛an｝的公比为q,前n项和为Sn,若S(n+1),Sn,S(n+2)成等差数列,则q= 设等比数列[an]的公比为q,前n项和为Sn,若S(n+1),Sn,S(n+2)成等差数列,则q的值? 设Sn为等差数列an的前n项和.求证Sn/n为等差数列等差数列{an}的前n项和为sn,a10 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 设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列设等差数列{an}的前n项和为Sn 若a1=Sn> 数学题设数列An等差数列前n项和为Sn若m不等于n,Sn=m平方,Sm=n平方,求S(m+n) 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