1.等差数列的首项为1,公差不为0.若a2,a3,a6成等比数列,则前6项的和为 ( )
A.-24 B.-3 C.3 D.8
2.已知{an}是等差数列,公差d不为零,前n项和是Sn,若a3,a4,a8成等比数列,则
( )
A.a1d>0,dS4>0 B.a1d<0,dS4<0
C.a1d>0,dS4<0 D.a1d<0,dS4>0
3.(2019·江苏高考)已知数列{an}(n∈N*)是等差数列,Sn是其前n项和.若a2a5+a8=0,S9=27,则S8的值是________.
4.设公差不为0的等差数列{an}的前n项和为Sn,若a2,a5,a11成等比数列,且a11=2(Sm-Sn)(m>n>0,m,n∈N*),
