问题 解答题
已知数列{an}满足a1=1,an+1=
an
2nan+1
(n∈N*).
(1)求通项an
(2)设A=
lim
n→+∞
3an
2an+1
,证明:对任意m≥2,且m∈N*,都有A∈>(1+
1
m
)m
答案

(1)由于a1=1,an+1=

an
2nan+1
,可得
1
an+1
-
1
an
=2n

1
a1
=1,
1
a2
-
1
a1
=2
1
a3
-
1
a2
=22
1
a4
-
1
a3
=23
,…,
1
an
-
1
an-1
=2n-1

累加可得

1
an
=1+2+22+23+…+2n-1=2n-1,故有an=
1
2n-1

(2)A=

lim
n→+∞
3an
2an+1
=
lim
n→∞
3(2n+1-1)
2(2n-1)
=
lim
n→∞
3(2-
1
2n
)
2(1-
1
2n
)
=
3×2
2
=3,

(1+

1
m
)m的通项公式为 Tr+1=
Ckn
1
mk
=
m(m-1)(m-2)…(m-k+1)
k!
1
mk
1
k!

(1+

1
m
)m
1
0!
+
1
1!
+
1
2!
+
1
3!
+…+
1
m!
 

≤1+1+

1
2×1
+
1
3×2
+
1
4×3
+…+
1
m(m-1)
=1+1+(1-
1
2
)+(
1
2
-
1
3
)+(
1
3
-
1
4
)+…+(
1
m-1
-
1
m

=1+1+(1-

1
m
)<3,

故对任意m≥2,且 m∈N*,都有A>(1+

1
m
)m

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