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=-
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(Ⅰ)∵bn+2=-bn+1-bn,
∴b3=-b2-b1=-3b1=3,
∴b1=-1.(3分)
(Ⅱ)∵bn+2=-bn+1-bn①
∴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=-
,b2=2b1=-1,b3=-3b1=1 2
,bn+3=bn,3 2
∴T1=b1=-
,T2=T1b2=1 2
,1 2
T3=T2b3=
,T4=T3b4=T3b1=3 4
T1,3 4
T5=T4b5=T2b3b4b5=T2b1b2b3=
T2,T6=T5b6=T3b4b5b6=T3b1b2b3=3 4
T3,3 4
…T3n+1+T3n+2+T3n+3=T3n-2b3n-1b3nb3n+1+T3n-1b3nb3n+1b3n+2+T3nb3n+1b3n+2b3n+3
=T3n-2b1b2b3+T3n-1b1b2b3+T3nb1b2b3=
(T3n-2+T3n-1+T3n),3 4
∴数列{T3n-2+T3n-1+T3n}(n∈N*)是等比数列
首项T1+T2+T3=
且公比q=3 4
(11分)3 4
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[1-(
[1-(3 4
)k]3 4 1- 3 4
)k]3 4
∴
≤Sn<3; (13分)3 4
②当n=3k-1(k∈N*)时Sn=(T1+T2+T3)+(T4+T5+T6)…+(T3k-2+T3k-1+T3k)-T3k
=3[1-(
)k]-(b1b2b3)k=3-4•(3 4
)k3 4
∴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-(
)k]-(b1b2b3)k-1b1b2-(b1b2b3)k=3[1-(3 4
)k]-3 4
(1 2
)k-1-(3 4
)k=3-3 4
•(14 3
)k3 4
∴-
≤Sn<3 (15分)1 2
综上得-
≤Sn<3则p≤-1 2
且q≥3,1 2
∴q-p的最小值为
. (16分)7 2