问题
解答题
在数列{an}中,a1=-
(I)求b1,b2; (II)证明数列{bn-1}是等比数列; (III)设cn=
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答案
(I)∵a1=-
,bn=an+n+1∴b1=a1+2=1 3 5 3
当n=2时,3a2-2a1+4=0可得a2=-14 9
∴b2=3+a2=13 9
(II)由3an-2an-1+n+2=0得,3(an+n)=2(an-1+n-1)
=an+n an-1+n-1
,n≥2即2 3
=bn-1 bn-1-1 2 3
∵b1- 1=
≠02 3
∴{bn-1}是以
为首项,2 3
为公比的等比数列2 3
(III)由(I)可得bn=
bn-1+2 3 1 3
∴2bn-1+1=3bn,所以bn=1+(
)n2 3
cn=
=(
)n2 3 2
+bnb 2n
=(
)n2 3 (2bn+1)bn
=bn-bn+1 bnbn+1
-1 bn+1 1 bn
Tn=C1+C2+…+Cn=
-1 bn+1
=1 b1
-3n+1 2n+1+ 3n+1 3 5