已知正项数列{an}的前n和为Sn,且Sn是14与(an+1)2的等比中项.(1)求证:数列{an}是等差数列;(2)若

2025-05-13 07:28:37
推荐回答(1个)
回答1:

(1)∵Sn

1
4
(an+1)2,∴a1S1
1
4
(a1+1)2
,∴a1=1(an>0)
当n≥2时,anSn?Sn?1
1
4
(an+1)2?
1
4
(an?1+1)2
,∴(an+an-1)(an-an-1-2)=0
∵an>0,
∴an-an-1=2,
∴{an}为等差数列.(4')
(2)由(1)知,{an}是以1为首项,2为公差的等差数列,
∴an=2n-1
bn
2n?1
2n
,①
Tn
1
2
+
3
22
+…+
2n?1
2n
,①
1
2
Tn=    
1
22
+
3
23
+
5
24
+…+
2n?3
2n
+
2n?1
2n

①-②得:
1
2
Tn
1
2
+2(
1
22
+
1
23
+
1
24
+
1
2n
)?
2n?1
2n+1

Tn=3?
2n?3
2n