|1/2009-1/2010|+|1/2010-1/2011|+|1/2011-1/2012|
=1/2009-1/2010+1/2010-1/2011+1/2011-1/2012
=1/2009-1/2012
=3/4042108最后的=3/4042108 是怎么得来的通分得到的1/2009-1/2012=(2012-2009)/(2009×2012)=3/4042108