{an}等差数列,若S4=21,a(n-3)+a(n-2)+a(n-1)+an=6,且Sn=286,则n=?
若Sn是公差不为0的等差数列{An}的前n项和,且S1,S2,S4成等比数列.an=2n-1.设bn=3/[an*a(n
等差数列an=2n-1,求数列{1/an*a n+1}的前n项和Sn
数列{an}中,a2=2,前n项和为sn,且sn=n(an+1)/2 证明{a-an}是等差数列
1.等差数列{an}前n项和为Sn,①S4≥10,S5≤15,求a4最大值;②a(2n)/an=(4n-1)/(2n-1
已知数列an的前n项和为sn=5/6n(n+3),1:求证an为等差数列 2:设bn=a3n+a
设Sn为等差数列{an}的前n项和,S7=42,Sn=510,若a(n-3)=45(n>7),则n等于
已知等差数列{an}的首项a1=a,公差d=2,前n项和为Sn,且S1,S2,S4成等比数列
已知等差数列{an}满足a(n+1)=an+3n+2,且a1=2,求an.
设数列{an}中,a1=1且(2n+1)an=(2n-3)a(n-1),(n大于等于2),求{an},sn
已知等差数列{an}中,a1+a2+a3=15,a(n-2)+a(n-1)+an=78,Sn=155,则n等于多少?
已知数列{an}的前n项和为Sn,a=1,an+1=2Sn+1(n∈N*),等差数列{bn}中,bn>0(n∈N*),且
已知等差数列{an}的前n项和Sn满足:Sn=n2+2n+a(n∈N*),则实数a=______.