问题 解答题
设M=
1
1+
2
+
1
2
+
3
+
1
3
+
4
+…+
1
2008
+
2009
,N=1-2+3-4+5-6+…+2009-2010.试求
N
(M+1)2+1
的值.
答案

∵M=

2
-1
(1+
2
)(
2
-1)
+
3
-
2
(
2
+
3
)(
3
-
2
)
+…+
2009
-
2008
(
2008
+
2009
)(
2009
-
2008
)

=

2
-1+
3
-
2
+
4
-
3
+…+
2009
-
2008

=

2009
-1,

N=(1-2)+(3-4)+(5-6)+…+=(-1)+(-1)+…+(-1)=-1005,

N
(M+1)2+1
=
-1005
(
2009
-1+1)
2
+1
=-
1
2

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