数列{an}的前n项和Sn=2n-1,则+++…+等于( )
(A)(2n-1)2(B)(2n-1)2
(C)4n-1(D)(4n-1)
等差数列{an}的前n项和为Sn,已知am-1+am+1-=0,S2m-1=38,则m=( )
(A)38(B)20(C)10(D)9
设等比数列{an}的各项均为正数,公比为q,前n项和为Sn.若对∀n∈N*,有S2n<3Sn,则q的取值范围是( )
(A)(0,1](B)(0,2)(C)[1,2)(D)(0,)
已知等差数列{an}的公差d≠0,且a1,a3,a9成等比数列,则=( )
(A)(B)(C)(D)
已知数列{an}的通项公式是an=2n-3()n,则其前20项和为( )
(A)380-(1-)(B)400-(1-)
(C)420-(1-)(D)440-(1-)
设等差数列{an}的前n项和为Sn,a2+a4=6,则S5等于( )
(A)10(B)12(C)15(D)30