已知正项数列{an}的前n项和为Sn,且a1=2,4Sn=an?an+1,n∈N*.(Ⅰ)求数列{an}的通项公式;(Ⅱ)设

2025-01-20 22:36:12
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回答1:

(Ⅰ)解:∵4Snan?an+1,n∈N* ①,
∴4a1=a1?a2
又a1=2,
∴a2=4.
当n≥2时,4Sn-1=an-1?an ②,
①-②得:4an=an?an+1-an-1?an
由题意知an≠0,
∴an+1-an-1=4,
当n=2k+1,k∈N*时,a2k+2-a2k=4,
即a2,a4,…,a2k是首项为4,公差为4的等差数列,
∴a2k=4+4(k-1)=4k=2×2k;
当n=2k,k∈N*时,a2k+1-a2k-1=4,
即a1,a3,…,a2k-1是首项为2,公差为4的等差数列,
∴a2k-1=2+4(k-1)=4k-2=2×(2k-1).
综上可知,an=2n,n∈N*
(Ⅱ)证明:∵

1
an2
1
4n2
1
4n(n+1)
1
4
(
1
n
?
1
n+1
),
Tn
1
a12
+
1
a22
+…+
1
an2
1
4
(1?
1
2
+
1
2
?
1
3
+…+
1
n
?
1
n+1
)

=
1
4
(1?
1
n+1
)=
n
4n+4

又∵
1
an2
1
4n2
1
4n2?1
1
(2n?1)(2n+1)
1
2
(
1
2n?1
?
1
2n+1
)

Tn
1
a12