1.等差数列{an}中,d=2,an=11,Sn=35,则a1为?

题目

1.等差数列{an}中,d=2,an=11,Sn=35,则a1为?
2.在项数为2n+1的等差数列中,所有奇数项的为165,所有偶数项的和为150,求n
3.等差数列{an}的前n项和为Sn,且6S5-5S3=5,求a4
4.设等差数列{an}的前n项和为Sn,若S9=72,a2+a4+a9=?

答案

1.an=a1+(n-1)d=a1+2n-2=11a1=13-2nSn=(a1+an)n/2=(24-2n)n/2=n(12-n)=35n^2-12n+35=0n=5或7a1=3或-12.S奇＝[a1+a(2n+1)](n+1)/2=165S偶＝[a2+a(2n)]n/2=150而a1+a(2n+1)=a2+a(2n)故：S奇/S偶＝(n+1)/n=165/1501/n=1...