问题
解答题
已知函数f(x)=
(1)求数列{an}的通项公式; (2)令Tn=a1a2-a2a3+a3a4-a4a5+…+a2n-1a2n-a2na2n+1,求Tn; (3)令bn=
|
答案
(1)∵an+1=f(
)=1 an
=an+2+3an 3 2 3
∴an+1-an=2 3
∴数列{an}是以
为公差,首项a1=1的等差数列2 3
∴an=
n+2 3 1 3
(2)Tn=a1a2-a2a3+a3a4-a4a5+…+a2n-1a2n-a2na2n+1
=a2(a1-a3)+a4(a3-a5)+…+a2n(a2n-1-a2n+1)
=-
(a2+a4+…+a2n)4 3
=-
×4 3 n(
+5 3
+4n 3
) 1 3 2
=-
(2n2+3n)4 9
(3)当n≥2时,bn=
=1 an-1an
=1 (
n-2 3
)(1 3
n+2 3
)1 3
(9 2
-1 2n-1
)1 2n+1
当n=1时,上式同样成立
∴sn=b1+b2+…+bn=
[(1-9 2
) +(1 3
-1 3
)+…+(1 5
-1 2n-1
)]1 2n+1
=
(1-9 2
)1 2n+1
∵恒有
(1-9 2
)<1 2n+1
成立,9 2
∵Sn<
,即m-2002 2
(1-9 2
)<1 2n+1
对一切n∈N*成立,m-2002 2
∴
≤9 2
,解得 m≥2011,m-2002 2
∴m最小=2011