等比数列An中,若Sn=12,S2n=24,求S3n答:a(n)=aq^(n-1),q=1时,s(n) = na.12 = s(n) = na,24 = s(2n) = 2na = 2*12,符合题意.此时,s(3n) = 3na = 3*12 = 36.q不为1时,s(n) = a[q^n-1]/(q-1),12 = s(n) = a[q^n - 1]/(q-1),24 = s(2n) = a[q^(2n)-1]/(q-1) = a[q^n...
什么是等差数列的意思概念介绍答:(2)在等差数列中,当项数为2n (n∈ N+)时,S偶-S奇 = nd,S奇÷S偶=an÷a(n+1);当项数为(2n-1)(n∈ N+)时,S奇—S偶=a(中),S奇-S偶=项数*a(中) ,S奇÷S偶 =n÷(n-1). (3)若数列为等差数列,则Sn,S2n -Sn ,S3n -S2n,…仍然成等差数列,公差为n^2d . (4)若数列{an}与{bn}...