已知等差数列{an}的公差d不等于0,其前n项和为sn,且a1,a3,a9成等比数列,则s3/s7=多少
来源:学生作业帮 编辑:作业帮 分类:数学作业 时间:2024/05/08 22:48:18
已知等差数列{an}的公差d不等于0,其前n项和为sn,且a1,a3,a9成等比数列,则s3/s7=多少
a(n) = a + (n-1)d,d不为0.
[a(3)]^2 = a(1)a(9) = a[a+8d] = [a+2d]^2,
0 = a^2 + 8ad - a^2 - 4ad - 4d^2 = 4ad - 4d^2 = 4d(a-d),
0 = a-d.
a = d.
a(n) = d + (n-1)d,
s(n) = nd + n(n-1)d/2 = (d/2)[2n + n(n-1)] = (d/2)[n^2 + n] = n(n+1)d/2
s(3)/s(7) = [3*4d/2]/[7*8d/2] = 3*4/(7*8) = 3/14
[a(3)]^2 = a(1)a(9) = a[a+8d] = [a+2d]^2,
0 = a^2 + 8ad - a^2 - 4ad - 4d^2 = 4ad - 4d^2 = 4d(a-d),
0 = a-d.
a = d.
a(n) = d + (n-1)d,
s(n) = nd + n(n-1)d/2 = (d/2)[2n + n(n-1)] = (d/2)[n^2 + n] = n(n+1)d/2
s(3)/s(7) = [3*4d/2]/[7*8d/2] = 3*4/(7*8) = 3/14
已知等差数列{an}的公差d不等于0,其前n项和为sn,且a1,a3,a9成等比数列,则s3/s7=多少
已知等差数列{an}的前n项和为Sn,公差d≠0,S5=4a3+6,且a1,a3,a9成等比数列.
已知等差数列{an}的前n项和为Sn,公差d≠0,且S3=9,a1,a3,a7成等比数列.
等差数列{an}的公差,d≠0,前n项和为Sn,且a1,a3,a9成等比数列,则S6/S10=
已知{an}是公差不为0的等差数列,a1=1,且a1,a3,a9成等比数列,求数列{2^an}的前n项和Sn
已知数列{an}是公差不为零的等差数列,其前n项和为Sn,且S5=30,又a1,a3,a9成等比数列.
已知等差数列{an}的公差d>0,其前n项和为Sn,若S3=12,且2a1,a2,1+a3成等比数列.(1)求{an}的
等差数列啊!设{an}是公差不为0的等差数列,a1=2,且a1,a3,a6成等比数列,则其前n项和Sn=?
已知等差数列{an}的前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13成等比数列.
已知{an}是等差数列,公差d不等于0,且a1 a3 a13成等比数列,sn是{an}的前n项和,(1)求证s1 s2
已知等差数列{an}前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列,求{an}通项公式.
已知等差数列{an}前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列 设{bn/an}是首