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=
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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
=2n2-n,n(1+4n-3) 2
(2)由(1)得,bn=
=an+1 Sn
=4n-2 2n2-n
,2 n
则bn•bn+1=
=4(4 n(n+1)
-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