1/7+2/3=17/21
1/2+1/3=5/6
3/4-3/8=3/8
1-7/9=2/9
2-1/6=11/6
2/5+3/5=1
5/6-2/3=1/6
4/5+1/6-4/5=1/6
7/8-5/12+5/6
=7/8+5/12
=31/24
4/5-(1/6+1/12)
=4/5-1/4
=11/20
2/5+(3/4+3/5+1/4)
=2/5+8/5
=2
4-8/13-5/13
=4-1
=3
4/5+(1/5+3/8)
=1+3/8
=11/8
17/20-(5/9-3/20)
=1-5/*9
4/9
不懂再问我我会说的详细点,