1991x19941994+1991x1991-1990x1992-1994x19911991
=1991×1994×10001-1994×1991×10001+1991×(1990+1)-1990×(1991+1)
=0+1991×1990+1991-1990×1991-1990
=1991×1990-1990×1991+1991-1990
=1
1991x19941994+1991x1991-1990x1992-1994x19911991
=1991×1994×10001-1994×1991×10001+1991×(1990+1)-1990×(1991+1)
=0+1991×1990+1991-1990×1991-1990
=1991×1990-1990×1991+1991-1990
=1