问题 解答题
已知递增的等差数列{an}满足:a2a3=45,a1+a4=14
(1)求数列{an}的通项公式及前n项和Sn
(2)设bn=
an+1
Sn
,求数列{bnbn+1}的前n项和Tn
答案

(1)由题意得,a1+a4=14,则a2+a3=14,

∵a2a3=45,∴a2、a3是方程x2-14x+45=0的两根,

∵等差数列{an}是递增数列,∴a2<a3

解得a2=5,a3=9,公差d=4,a1=1,

∴an=4n-3,

Sn=

n(a1+an)
2
=
n(1+4n-3)
2
=2n2-n,

(2)由(1)得,bn=

an+1
Sn
=
4n-2
2n2-n
=
2
n

则bn•bn+1=

4
n(n+1)
=4(
1
n
-
1
n+1
),

∴Tn=b1•b2+b2•b3+…+bn•bn+1

=4[(1-

1
2
)+(
1
2
-
1
3
)+…+(
1
n
-
1
n+1
)]

=4(1-

1
n+1
)=
4n
n+1

单项选择题
单项选择题