问题 解答题
设数列{bn}满足bn+2=-bn+1-bn,(n∈N*),b2=2b1
(I)若b3=3,求b1的值;
(II)求证数列{bnbn+1bn+2+n}是等差数列;
(III)设数列{Tn}满足:Tn+1=Tnbn+1(n∈N*),且T1=b1=-
1
2
,若存在实数p,q,对任意n∈N*都有p≤T1+T2+T3+…+Tn<q成立,试求q-p的最小值.
答案

(Ⅰ)∵bn+2=-bn+1-bn

∴b3=-b2-b1=-3b1=3,

∴b1=-1.(3分)

(Ⅱ)∵bn+2=-bn+1-bn

∴bn+3=-bn+2-bn+1②,

②-①得bn+3=bn (5分)

∴(bn+1bn+2bn+3+n+1)-(bnbn+1bn+2+n)=bn+1bn+2(bn+3-bn)+1=1为常数

∴数列{bnbn+1bn+2+n}是等差数列. (7分)

(Ⅲ)∵Tn+1=Tn•bn+1=Tn-1bnbn+1=Tn-2bn-1bnbn+1=…=b1b2b3…bn+1

当n≥2时Tn=b1b2b3…bn(*),当n=1时T1=b1适合(*)式

∴Tn=b1b2b3…bn(n∈N*). (9分)

b1=-

1
2
,b2=2b1=-1,b3=-3b1=
3
2
,bn+3=bn

T1=b1=-

1
2
T2=T1b2=
1
2

T3=T2b3=

3
4
T4=T3b4=T3b1=
3
4
T1

T5=T4b5=T2b3b4b5=T2b1b2b3=

3
4
T2T6=T5b6=T3b4b5b6=T3b1b2b3=
3
4
T3

…T3n+1+T3n+2+T3n+3=T3n-2b3n-1b3nb3n+1+T3n-1b3nb3n+1b3n+2+T3nb3n+1b3n+2b3n+3

=T3n-2b1b2b3+T3n-1b1b2b3+T3nb1b2b3=

3
4
(T3n-2+T3n-1+T3n),

∴数列{T3n-2+T3n-1+T3n}(n∈N*)是等比数列

首项T1+T2+T3=

3
4
且公比q=
3
4
 (11分)

记Sn=T1+T2+T3+…+Tn

①当n=3k(k∈N*)时,Sn=(T1+T2+T3)+(T4+T5+T6)…+(T3k-2+T3k-1+T3k)=

3
4
[1-(
3
4
)
k
]
1-
3
4
=3[1-(
3
4
)k]

3
4
Sn<3; (13分)

②当n=3k-1(k∈N*)时Sn=(T1+T2+T3)+(T4+T5+T6)…+(T3k-2+T3k-1+T3k)-T3k

=3[1-(

3
4
)k]-(b1b2b3)k=3-4•(
3
4
)k

∴0≤Sn<3; (14分)

③当n=3k-2(k∈N*)时Sn=(T1+T2+T3)+(T4+T5+T6)…+(T3k-2+T3k-1+T3k)-T3k-1-T3k

=3[1-(

3
4
)k]-(b1b2b3)k-1b1b2-(b1b2b3)k=3[1-(
3
4
)k]
-
1
2
(
3
4
)k-1
-(
3
4
)k
=3-
14
3
•(
3
4
)k

-

1
2
Sn<3 (15分)

综上得-

1
2
Sn<3则p≤-
1
2
且q≥3,

∴q-p的最小值为

7
2
. (16分)

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