1乘3分之1+3乘5分之1+5乘7分之1……2007*2009分之1
=(1-1/3)/2+(1/3-1/5)/2+……+(1/2007-1/2009)/2
=(1-1/3+1/3-1/5+……+1/2007-1/2009）/2
=（1-1/2009）/2
=1004/2009