去绝对值符号呀
|1/3-1/2|+|1/4-1/3|+|1/5-1/6|+...+|1/2009-1/2008|
=1/2-1/3+1/3-1/4+...+1/2008-1/2009
=1/2-1/2009
=2007/4018