已知数列{an}的前n项和为Sn,且满足Sn=2an-1(n∈N*),数列{bn}满足b1=1,nbn+1=(n+1)bn,(n∈N*)(

2025-04-07 11:58:08
推荐回答(1个)
回答1:

(1)令n=1,得a1=S1=2a1-1,解得a1=1,
当n≥2时,an=Sn-Sn-1=2(an-an-1),
整理,得an=2an-1
an2n?1
∵数列{bn}满足b1=1,nbn+1=(n+1)bn

bn+1
n+1
bn
n

∴{
bn
n
}是首项为1的常数列,∴
bn
n
=1

∴bn=n.
(2)∵数列{bn}的前n项和为Qn
Qn=1+2+3+…+n=
n(n+1)
2

∵Tn=Sn+Qn
∴Tn=2?2n?1?1+
n(n?1)
2
=2n?1+
n(n+1)
2

当n=1时,λT1≥T2,得λ≥3,
当n=2时,λT2≥T3,得λ≥
13
6

猜想:当λ≥3时,3Tn≥Tn+1
证明:3Tn?Tn+1=3[2n?1+
n(n+1)
2
]
-[2n+1?1+
(n+1)(n+2)
2
]

=2n+n-3≥0.
综上所述,λ存在最小值3,使不等式λTn≥Tn+1成立.