(1995.5-1993.5)/1/1998*1999又1997/1998
=2*1998*(1999+1997/1998)
=2*1998*(1999*1998-1997)/1998
=2*1999*1998-2*1997
=2*1999(1997+1)-2*1997
=2*1999*1997+2*1999-2*1997
=2*1999*1997+2*(1997+2)-2*1997
=2*1999*1997+4
=2*(1998+1)(1998-1)+4
=2*1998²-2+4
=2*1998²+2