1/2004+3/2004+5/2004+...+2003/2004=(1/2004+2003/2004)+(3/2004+2001/2004)+...+(1001/2004+1003/2004)=1*501(1-2003为1002个数,1002个数中有2个一组,最后为1002/2=501组)=501