已知在数列{an}中,a1=1,a2n+1=qa2n-1+d(d∈R,q∈R 且q≠0,n∈N*).(1)若数列{a2n-1}是等比数列,

2025-05-12 21:19:49
推荐回答(1个)
回答(1):

(1)∵a1=1,a2n+1=qa2n-1+d,q≠0,
①当d=0时,a2n+1=qa2n-1,显然{a2n-1}是等比数列;
②当d≠0时,a3=qa1+d=q+d,a5=qa3+d=q(q+d)+d.
∵数列{a2n-1}是等比数列,

a
a1a5,即(q+d)2=q(q+d)+d,化简得q+d=1.
此时有a2n+1=qa2n-1+1-q,得a2n+1-1=q(a2n-1-1),
由 a1=1,q≠0,得a2n-1=1(n∈N*),则数列{a2n-1}是等比数列.
综上,q与d满足的条件为d=0(q≠0)或q+d=1(q≠0,d≠0).
(2)当d=0,q=2时,
∵a2n+1=2a2n-1
a2n?1a1?2n?12n?1
依题意得:x4=a1-a3=1-2,x8=1?2+22?23,…,
x4n=1?2+22?23+…+22n?2?22n?1
1?(?2)2n
1?(?2)
1?22n
1+2
1?22n
3

1?3x4n22n
x4n
1?22n
3

∴Sn=x4+2x8+3x12+…+(n-1)?x4(n-1)+n?x4n=
1
3
(1+2+3+…+n)?
1
3
(1×22+2×24+3×26+…+n?22n)
=
n(n+1)
6
?
1
3
(1×22+2×24+3×26+…+n?22n)

Tn=1×22+2×24+3×26+…+(n?1)?22(n?1)+n?22n
4Tn=1×24+2×26+3×28+…+(n-1)?22n+n?22n+2
①-②得?3Tn=1×22+24+26+…+22n?n?22n+2=
22(1?22n)
1?4
?n?22n+2
=
4
3
(22n?1)?n?22n+2

Tn
4
9
(1?22n)+
n?22n+2
3
4
9
+
(3n?1)?22n+2
9

Sn
n(n+1)
6
?
4
27
?
(3n?1)?22n+2
27