因为分子相同时,分母越大的数月小所以1/2010<1/2009,1/2009<1/2008,1/2008<1/2007,1/2010<1/2007
=1/2009-1/2010+1/2008-1/2009+1/2007-1/2008+1/2010-1/2007
=0