1996×1/1997+1997×1/1998+1998×1/1999+1/1999
=(1-1/1997)+(1-1/1998)+(1-1/1999)+1/1999
=3-(1/1997+1/1998)
=3-(1997+1998)/(1997×1998)
=3-3995/3990006
=2又3986001/3990006