问题 填空题
数列{an}满足a1>1,an+1-1=an(an-1),(n∈N+),且
1
a1
+
1
a2
+…+
1
a2012
=2,则a2013-4a1的最小值为______.
答案

a1>1,由an+1-1=an(an-1),(n∈N+)知,对所有n,an>1,

等式两边取倒数,得

1
an+1-1
=
1
an(an-1)
=
1
an-1
-
1
an
,得,

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

1
a1
+
1
a2
+…+
1
a2012
=
1
a1
-
1
a2013
=2

整理可得,a2013=

2-a1
3-2a1

a2013-4a1=2(3-2a1)+

1
2(3-2a1)
-
11
2
≥2
(3-2a1)
1
3-2a1
-
11
2
=-
7
2

则a2013-4a1的最小值为 -

7
2

故答案为:-

7
2

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