1. 2又1/2-3/4+3/7
=1又3/4+3/7
=2又5/28
2. 3又 5/12+1/4-2 又2/3
=（3又5/12-2又2/3）+1/4
=3/4+1/4
=1
3. 3 又3/4-3又 3/5+4又 1/12
=（3又3/4+4又1/12）-3又3/5
=7又5/6-3又3/5
=4又7/30
4.12又 1/3-2又5/6+1/15
=9又1/2+1/15
=9又17/30
5.8 又3/10-2又1/5+1/2
=6.1+0.5
=6.6