问题 选择题
设数列{an}满足a1=
1
3
,an+1=an2+an(n∈N*),记Sn=
1
1+a1
+
1
1+a2
+…+
1
1+an
,则S10的整数部分为(  )
A.1B.2C.3D.4
答案

∵数列{an}满足a1=

1
3
,an+1=an2+an=an(an+1)(n∈N*),

1
an+1
=
1
an(an+1)
=
1
an
×
1
an+1
=
1
an
-
1
an+1

1
an+1
=
1
an
-
1
an+1

S10=

1
a1
-
1
a2
+
1
a2
-
1
a3
+…+
1
a10
-
1
a11
=
1
a1
-
1
a11

a1=

1
3

a2=

1
9
+
1
3
 =
4
9

a3=

16
81
+
4
9
=
52
81

a4=

2704
6561
+
52
81
>1,

又an+1>an

∴a11>1,

∴0<

1
a11
<1,

1
a1
=3,

∴S10的整数部分是2.

故选B.

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