x 2
1+ x 2 =
1
1
x 2 +1 …(1)
x 2
1+ x 2 = 1-
1
1+ x 2 …(2)
当x分别等于
1
2005 ,
1
2004 ,
1
2003 ,
1
2002 ,
1
2001 ,
1
2000 时,代入(1)式并相加得:
1
2005 2 +1 +
1
2004 2 +1 +
1
2003 2 +1 +
1
2002 2 + 1 +
1
2001 2 +1 +
1
2000 2 +1 …(3)
当x分别等于2000,2001,2002,2003,2004,2005时,代入(2)式并相加得: 1-
1
1+ 2000 2 +1-
1
1+ 2001 2 +1-
1
1+ 2002 2 +1-
1
1+ 2003 2 +1-
1
1+ 2004 2 +1-
1
1+ 2005 2 …(4)
将(3)式和(4)式相加得:6.
故答案为:6.