=(1999*1997+1)/1997 * 1997/1996 = (1999*1997+1)/1996 = (1998^2-1+1)/1996=(1998^2)/1996
其实，把题目换成(1999*1/1997)*1997/1998更好
=(1999*1997+1)/1997 * 1997/1996
= (1999*1997+1)/1996
=1999*(1996+1)/1996+1/1996
=1999+1999/1996+1/1996
=1999+2000/1996
=2000+4/1996
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