(1-1/2)(1/3-1)(1-1/4)(1/5-1).(1-1/2004)(1/2005-1)

(1-1/2)(1/3-1)(1-1/4)(1/5-1).(1-1/2004)(1/2005-1)
(1-1/2)(1/3-1)(1-1/4)(1/5-1).(1-1/2004)(1/2005-1)

(1-1/2)(1/3-1)(1-1/4)(1/5-1).(1-1/2004)(1/2005-1)
=（1/2）（-2/3)(3/4)(-4/5).(2003/2004)(-2004/2005)
=1/2005
重要的是要算出有多小个负数
2004/2=1002个负数,所以得数为正数