3分之1-7分之2
=7/21-6/21
=1/21
6分之5+12分之5
= 10/12+5/12
=15/12
=5/4
12分之7+4分之1
= 7/12+3/12
=10/12
=5/6
2分之1-6分之1
=3/6-1/6
=2/6
=1/3