1999÷1998/1999
=1999×1999/1998
=(1998+1)×1999/1998
=1998×1999/1998+1×1999/1998
=1999+1999/1998
=1999+1+1/1998
=2000+1/1998
=2000又1998分之1
1999÷1998/1999
=1999×1999/1998
=(1998+1)×1999/1998
=1998×1999/1998+1×1999/1998
=1999+1999/1998
=1999+1+1/1998
=2000+1/1998
=2000又1998分之1