问题
解答题
已知数列{an}满足a1=1,an+1=
(1)求通项an; (2)设A=
|
答案
(1)由于a1=1,an+1=
,可得an 2nan+1
-1 an+1
=2n,1 an
∴
=1,1 a1
-1 a2
=2,1 a1
-1 a3
=22,1 a2
-1 a4
=23,…,1 a3
-1 an
=2n-1,1 an-1
累加可得
=1+2+22+23+…+2n-1=2n-1,故有an=1 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
=3,3×2 2
而(1+
)m的通项公式为 Tr+1=1 m
•C kn
=1 mk
•m(m-1)(m-2)…(m-k+1) k!
≤1 mk
,1 k!
∴(1+
)m≤1 m
+1 0!
+1 1!
+1 2!
+…+1 3! 1 m!
≤1+1+
+1 2×1
+1 3×2
+…+1 4×3
=1+1+(1-1 m(m-1)
)+(1 2
-1 2
)+(1 3
-1 3
)+…+(1 4
-1 m-1
)1 m
=1+1+(1-
)<3,1 m
故对任意m≥2,且 m∈N*,都有A>(1+
)m.1 m