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=
|
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⇒
=2(n≥2)an+1 an
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)
=2(2k (2k-1+1)(2k+1)
-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)
=2[(2n (an+1)(an+1+1)
-1 20+1
)+(1 2+1
-1 2+1
)+(1 22+1
-1 22+1
)++(1 22+1
-1 2n-1+1
)]=2(1 2n+1
-1 2
)1 2n+1
∵0<
≤1 2n+1
,∴1 3
≤2(1 3
-1 2
)<1,1 2n+1
所以
≤Tn<11 3