a+x=1999,b+x=2000,c+x=2001,且abc=12
a+2=b+1=c
12=2*3*4
a=2
b=3
c=4
a/bc + b/ac + c/bc - 1/a - 1/b - 1/c
=2/12+3/8+4/6-1/2-1/3-1/4
=1/6+3/8+4/6-5/6-1/4
=1/8