等差数列{an}的前n项和为Sn,且S4=20,Sn-4=60,Sn=120,则n=______.
等差数列{an}的前n项和为Sn,且S4=20,Sn-4=60,Sn=120,则n=______.
若Sn是公比为q的等比数列{an}的前n项和,且S4,S6,S5成等差数列,则公比q=______.
记等差数列{an}的前n项和为Sn,若S2=4,S4=20,则该数列的公差d=______.
已知等差数列{an}的前n项和为Sn,且S4=3,S8=7,则S12=
等差数列{an}的前n项和为Sn,已知S4=S5=-20,若Sn>0则n的最小值为?
已知等差数列{an}的前n项和为Sn,且(2n-1)Sn+1 -(2n+1)Sn=4n²-1(n∈N*)
设等差数列an的前n项和为Sn,若S4>=10,S5
已知等差数列{an}的前n项和为Sn,a3=0,S4=-4.
设等差数列{an}的前n项和为Sn,且S4=4S2,a4=2a2+1,求数列{an}的通项公式及前n项和Sn.
已知数列{an}的前n项和为Sn,且满足Sn=Sn-1/2Sn-1 +1,a1=2,求证{1/Sn}是等差数列
设等差数列{an}的前n项和为Sn,若a1>0,S4=S8,则当Sn取得最大值时,n的取值为?
设等差数列an的前n项和为Sn,a1=3/2,且s1,s2,s4成等比数列,求sn