问题 解答题
已知各项均为正数的数列﹛an﹜,对于任意正整数n,点(an,sn)在曲线y=
1
2
(x2+x)

(1)求证:数列﹛an﹜是等差数列;
(2)若数列﹛bn﹜满足bn=
1
anan+2
,求数列﹛bn﹜的前n项和Tn
答案

(1)∵各项均为正数的数列﹛an﹜,对于任意正整数n,点(an,sn)在曲线y=

1
2
(x2+x)上,

Sn=

1
2
(an2+an),①

∴Sn-1=

1
2
an-12+an-1),n≥2,②

①-②,得an=Sn-Sn-1=

1
2
[(an2+an)-(an-12+an-1)]

an-12+an-1=an2-an

an2-an-12=an+an-1

∴an-an-1=1.

∴数列﹛an﹜是等差数列.

(2)∵Sn=

1
2
(an2+an),

a1=

1
2
(a12+a1),解得a1=1,a1=0(舍),

∵an-an-1=1.

∴an=1+(n-1)=n,

∴bn=

1
anan+2
=
1
n(n+2)
=
1
2
(
1
n
-
1
n+2
)

∴数列﹛bn﹜的前n项和

Tn=b1+b2+b3+…+bn

=

1
2
(1-
1
3
)+
1
2
(
1
2
-
1
4
)
+
1
2
1
3
-
1
5
)+…+
1
2
(
1
n
-
1
n+2
)

=

1
2
(1+
1
2
-
1
n+1
-
1
n+2

=

3
4
-
1
2n+2
-
1
2n+4

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