1/2+3/4-6/5
=10/20+15/20-24/20
=1/20
6/7-1/2+4/5
=5/14+4/5
=81/70
3/4-(7/12-5/7)
=3/4-7/12+5/7
=1/6+5/7
=37/42
2/3+(13/15+1/3)
=2/3+1/3+13/15
=1+13/15
=1又13/15
15/9-2/7-5/7
解方程
x+5/6=5/2
x=5/2-5/6
x=5/3
x-3/4=2/3
x=2/3+3/4
x=17/12
9/7-x=2/5
x=9/7-2/5
x=31/35