在公差为d的等差数列{an}中,已知a1=10,且a1,2a2+2,5a3成等比数列.
(1)求d,an;
(2)若d<0,求|a1|+|a2|+…+|an|.
已知公差不为零的等差数列{an}的前4项和为10,且a2,a3,a7成等比数列.
(1)求通项公式an;
(2)设bn=2an,求数列{bn}的前n项和Sn.
设Sn为数列{an}的前n项和,Sn=(-1)nan-,n∈N*,则:
(1)a3=________;
(2)S1+S2+…+S100=________.
设数列{an}是公差不为0的等差数列,a1=1且a1,a3,a6成等比数列,则数列{an}的前n项和Sn=________.
若等比数列{an}满足a2+a4=20,a3+a5=40,则公比q=________;前n项和Sn=________.
设等差数列{an}的前n项和为Sn,Sm-1=-2,Sm=0,Sm+1=3,则m等于( ).
A.3 B.4 C.5 D.6