等差数列{an}中,Sn=18,an+an-1+an-2=3,S3=1
来源:学生作业帮 编辑:作业帮 分类:数学作业 时间:2024/05/11 01:10:29
等差数列{an}中,Sn=18,an+an-1+an-2=3,S3=1
n=?
n=?
设an=a+(n-1)d
Sn=[a+a+(n-1)d]xn/2=[2a+(n-1)d]xn/2=18
an+an-1+an-2=a+(n-1)d+a+(n-2)d+a+(n-3)d=3a+(3n-6)d=3
S3=a+a+d+a+2d=3a+3d=1
由3a+(3n-6)d=3和3a+3d=1解得
a=(n-5)/(3n-9) d=2/(3n-9)
将a、d代入[2a+(n-1)d]xn/2=18
[(2n-10)/(3n-9)+(2n-2)/(3n-9)]xn/2=18
[(4n-12)/(3n-9)]xn/2=18
4/3xn/2=18
n=27
Sn=[a+a+(n-1)d]xn/2=[2a+(n-1)d]xn/2=18
an+an-1+an-2=a+(n-1)d+a+(n-2)d+a+(n-3)d=3a+(3n-6)d=3
S3=a+a+d+a+2d=3a+3d=1
由3a+(3n-6)d=3和3a+3d=1解得
a=(n-5)/(3n-9) d=2/(3n-9)
将a、d代入[2a+(n-1)d]xn/2=18
[(2n-10)/(3n-9)+(2n-2)/(3n-9)]xn/2=18
[(4n-12)/(3n-9)]xn/2=18
4/3xn/2=18
n=27
等差数列{an}中,a1+a2+a3=21,an-2+an-1+an=57,Sn=520,求n.
已知等差数列{an}中,a1=2.an+1=an+3分之an 求an
在等差数列an中,a1=2,3an+1-an=0,求an
数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn
数列{an}中,已知sn=an-1/sn-2,①:求出s1,s2,s3,s4,②:猜想数列{an}的前n项和sn的公式,
已知等差数列[An],Sn=[(An+1)/2]^2,求An的通项公式
在以d为公差的等差数列an中,设S1=a1+a2.+an,S2=an+1+an+2+a2n,S3=a2n+1+a2n+a
已知数列{an}中,a2=2,前n项和为Sn,且Sn=n(an+1)/2证明数列{an+1-an}是等差数列
会的进.等差数列的题1.在等差数列中{An} A1+A2+A3=15 ,An+An-1+An-2=78,Sn=155 求
等差数列an的前n项和为sn,a1=1+根号2,s3=9+3根号2
已知{an}是等差数列,a1+a2+a3+a4=11,an-3+an-2+an-1+an=67且sn=286求项数n?
已知数列{an}的前n项和Sn满足Sn=2an-1,等差数列{bn}满足b1=a1,b4=S3.