设Sn是等差数列{an}的前N项和,a14=-6,s9=-18,求s18
设Sn是等差数列{an}的前n项和,a12=-8,S9=-9,﹛Sn/n﹜是等差数列,求﹛Sn/n﹜的前n项的和
设Sn是等差数列{an}的前n项和,若a5/a3=5/9,则S9/S5=:
设Sn是等比数列{an}的前n项和,S3,S9,S6成等差数列.
设Sn为等差数列的前n项和,且S9=18,Sn=240,若an-4=30求项数n
设等差数列{an}的前n项和为Sn,且a5=3a7,则S9/S13=?
设等差数列[an]的前n项和为Sn,若a5=5a3,则S9/S5等于多少?
设等差数列{an}的前n项和为Sn,已知a4=12,且S8>0,S9
{An}为等差数列,前n项和为Sn,且S4=-62,S6=-75,求|a1|+|a2|+…+|a14|的值. 解:设Ak
设Sn是等差数列的{an}前n项和,且S9=18,Sn=240,若an-4=30(n>9),则n的值为( )
设Sn是等比数列an的前n项和S3.S9.S6成等差数列,求a3.a9.a6成等差数列
设等差数列 {an}的前n 项和为Sn,若S9>0 ,S10
设Sn是等差数列{an}的前n项和,a12=-8,S9=-9,则S16=______.