问题 解答题
设数列{an}的前n项和为Sn,且a1=1,Sn=an+1-1.
(1)求数列{an}的通项公式.(2)设bn=
2n
(an+1)(an+1+1)
Tn=b1+b2+
…+bn,求证:
1
3
Tn
<1
答案

(1)∵an+1-Sn-1=0①∴n≥2时,an-Sn-1-1=0②

①-②得:((an+1-an)-(Sn-Sn-1)=0⇒(an+1-an)-an=0⇒

an+1
an
=2(n≥2)

由an+1-2Sn-1=0及a1=1得a2-S1-1=0⇒a2=S1+1=a1+1=2∴{an}是首项为1,公比为2的等比数列,

∴an=2n-1

(2)∵bk=

2k
(ak+1)(ak+1+1)
=
2k
(2k-1+1)(2k+1)
=2(
1
2k-1+1
-
1
2k+1
)

∴Tn=b1+b2+…+bn=

2
(a1+1)(a2+1)
+
22
(a2+1)(a3+1)
+
23
(a3+1)(a4+1)
++
2n
(an+1)(an+1+1)
=2[(
1
20+1
-
1
2+1
)+(
1
2+1
-
1
22+1
)+(
1
22+1
-
1
22+1
)++(
1
2n-1+1
-
1
2n+1
)]
=2(
1
2
-
1
2n+1
)

∵0<

1
2n+1
1
3
,∴
1
3
≤2(
1
2
-
1
2n+1
)<1,

所以

1
3
Tn<1

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