已知数列an满足a1=1,a3+a7=18

来源：学生作业帮助网编辑：六六作业网时间：2024/11/28 05:10:00

已知数列an满足a1=1,a3+a7=18已知数列an满足a1=1,a3+a7=18 已知数列an满足a1=1,a3+a7=18a1=1,a3+a7=18,a(n+1)+a(n-1)=2an

已知数列an满足a1=1,a3+a7=18
已知数列an满足a1=1,a3+a7=18

已知数列an满足a1=1,a3+a7=18
a1=1,a3+a7=18,
a(n+1)+a(n-1)=2an
a(n+1)-an = an - a(n-1)
a(n+1)-an= a7-a6 (1)
a(n+1)-an= a6-a5 (2)
a(n+1)-an= a5-a4 (3)
a(n+1)-an= a4-a3 (4)
a(n+1)-an= a3-a2 (5)
a(n+1)-an= a2-a1 (6)
(1)+(2)+(3)+(4)+2(5)+2(6)
8[a(n+1)-an]= a7+a3- 2a1
=18 -2
a(n+1)-an=2
an -a1=2(n-1)
an = 2n-1
(2)
let
S= 1.2^1+2.2^2+...+n.2^n (1)
2S= 1.2^2+2.2^3...+n.2^(n+1) (2)
(2)-(1)
S= n.2^(n+1) -(2+2^2+...+2^n)
=n.2^(n+1) -2(2^n -1)
cn=2^(n-1).an
= 2^(n-1).( 2n-1)
= n.2^n - 2^(n-1)
Tn=c1+c2+...+cn
=S - (2^n-1)
= n.2^(n+1) -3(2^n -1)
= 3+ (2n-3).2^n

已知数列an满足a1=1,a3+a7=18 已知数列,an满足a1=1,a3+a7=18,且an-1+an+1=2an 已知数列{an}满足a1=5,anan+1=2的n次方,则a7/a3= 已知公差不为0的等差数列｛an｝满足a2=3,a1,a3,a7成等比数列 (1)求an的通项公式已知公差不为0的等差数列｛an｝满足a2=3,a1,a3,a7成等比数列(1)求an的通项公式数列{bn}满足bn=an/an+1+an+1/an,求数列bn前n 已知数列{an}满足a1=1,a3+a7=18,且an-1+an+1=2an(n大于等于2) (1)求数列(an)的通项已知数列{an}满足a1=1,a3+a7=18,且an-1+an+1=2an(n大于等于2)(1)求数列(an)的通项公式(2)若Cn=2n-1*an,求数列{Cn}的前n项和T 巳知数列{an}满足a1=5,anan+1=2^n,则a7/a3=, 已知公差不为0的等差数列an满足a2=3,a1、a3、a7成等比数列.（1）求an通项公式（2）已知公差不为0的等差数列an满足a2=3,a1、a3、a7成等比数列.（1）求an通项公式（2）数列bn满足：bn=[（an／an+1）+ 已知等差数列{an}中,a1=1,a7=4,数列{bn}是等比数列,b1=6,b2=a3.满足a26bna26 X bn 急用!数列{an}满足a1=1,a3+a7=18.已知数列{an}满足a1=1,a3+a7=18,且a下标n-1+a下标n+1=2下标an.求数列{an}通项公式.若cn=2上标n-1 已知数列{an}满足a1=1,a3+a7=18,且an-1+an+1=2an(n>=2).(Ⅰ)求数列{an}的通项公式；(Ⅱ)若cn=2n-1 已知等比数列{an}满足a1+a2=3,a2+a3=6,求这个数列的第7项a7 已知数列an'满足a1=1/2,a1+a2+a3+...+an=n^2an,求通项公式已知数列{An}满足A1=1,A3+A7=18,且A（n-1）+A（n+1）=2An若Cn=2^n-1*An,求{Cn}前N项和如题已知数列an满足a1=5 an乘an+1=2^n 则a7/a3=A．2 B．4 C．5 D．5/2 已知数列{an}满足a1=1 Sn=n² 则a3的值为几个数列问题.已知数列{an} a1=1,an+1=an/(1+n^2*an) 求an 已知数列{an} 满足a1=1 a1*a2*a3.*an=n^2 求an 已知数列an是公比q=1/2的等比数列且a1+a4+a7+...a100=60 求a1+a2+a3+...a102 已知等差数列{an}公差为d,满足a1+a2+a3=15,且a1+1,a3+1,a7+1构成等你数列的连续三项（1）求数列{an}的通项公式（2）若等差数列{an}递增,其前n项和为Tn,求证1/3=