问题 解答题
已知正项数列{an}满足a1=P(0<P<1),且an+1=
an
1+an
n∈N*
(1)若bn=
1
an
,求证:数列{bn}为等差数列;
(2)求证:
a1
2
+
a2
3
+
a3
4
+…+
an
n+1
<1
答案

(1)b1=

1
a1
=
1
P

bn+1-bn=

1
an+1
-
1
an
=
1+an
an
-
1
an
=1& 

故数列{bn}是以b1=

1
P
为首项,以1为等差的等差数列              

(2)证明:bn=

1
an
=
1
p
+(n-1)⇒an=
1
n+
1
p
-1

0<p<1

 ∴
1
p
-1>0
an=
1
n+
1
p
-1
1
n

a1
2
+
a2
3
+
a3
4
+…+
an
n+1
1
2×1
+
1
3×2
+
1
4×3
+…+
1
(n+1)n

=1-

1
2
+
1
2
-
1
3
+
1
3
-
1
4
+…+
1
n
-
1
n+1

=1-

1
n+1
<1

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