(1/2-1)(1/3-1)(1/4-1)...(1/2010-1)(1/2011-1)(1/2012-1)换算一下：
=(-1/2)(-2/3)(-3/4)...(-2009/2010)(-2010/2011)(-2011/2012),共有2011个负数相乘
=-(1/2X2/3X3/4X...X2009/2010X2010/2011X2011/2012),然后开始约分,最终剩下
=-1/2012