12+1又3/4-8又5/12-6.75
=12+1.75-8又5/12-6.75
=12+1.75-6.75 -8又5/12
=7-8又5/1=-1又5/12
|12.7-（-8.9）|=|12.7+8.9|=|21.6|=21.6