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)求证:数列﹛an﹜是等差数列; (2)若数列﹛bn﹜满足bn=
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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=
(x2+x)上,1 2
∴Sn=
(an2+an),①1 2
∴Sn-1=
(an-12+an-1),n≥2,②1 2
①-②,得an=Sn-Sn-1=
[(an2+an)-(an-12+an-1)]1 2
∴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=
(an2+an),1 2
∴a1=
(a12+a1),解得a1=1,a1=0(舍),1 2
∵an-an-1=1.
∴an=1+(n-1)=n,
∴bn=
=1 an•an+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
=
(1-1 2
)+1 3
(1 2
-1 2
)+1 4
(1 2
-1 3
)+…+1 5
(1 2
-1 n
)1 n+2
=
(1+1 2
-1 2
-1 n+1
)1 n+2
=
-3 4
-1 2n+2
.1 2n+4