问题
解答题
设{an}是正数数列,其前n项和Sn满足Sn=
(1)求a1的值; (3)求数列{an}的通项公式; (5)对于数列{bn},Tn为数列{bn}的前n项和,令bn=
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答案
(1)由a1=S1=
(a1-1)(a1+3),及an>0,得a1=31 4
(2)由Sn=
(an-1)(an+3)1 4
得Sn-1=
(an-1-1)(an-1+3).∴当n≥2时,1 4
an=
(1 4
-a 2n
)+2(an-an-1)a 2n-1
∴2(an+an-1)=(an+an-1)(an-an-1)
∵an+an-1>0∴an-an-1=2,
∴由(1)知,{an}是以3为首项,2为公差的等差数列,∴an=2n+1.
(3)由(2)知Sn=n(n+2)∴bn=
=1 Sn
(1 2
-1 n
),1 n+2
Tn=b1+b2+…+bn
=
(1-1 2
+1 3
-1 2
++1 4
-1 n-1
+1 n+1
-1 n
)1 n+2
=
[1 2
-3 2
]2n+3 (n+1)(n+2)
=
-3 4 2n+3 2(n+1)(n+2)