17/18-(1/3+1/18)
=17/18-1/3-1/18
=17/18-1/18-1/3
=8/9-3/9
=5/9

5-1/4-2/5-3/4
=5-(1/4+3/4)-2/5
=4-2/5
=1又3/5
5/8+5/7-5/7+5/8
=5/8+5/8+(5/7-5/7)
=10/8
=5/4
4/15+5/12+11/15
=4/15+11/15+5/12
=1+5/12
=1又5/12