7分之5-14分之3+4分之1
=10/14 - 3/14 +1/4
=1/2+1/4
=3/4
4分之9-（4分之3+3分之2）
= 9/4-3/4-2/3
=6/4-2/3
=3/2-2/3
=9/6-4/6
=5/6
8分之7-9分之1-4分3
=7/8-3/4-1/9
=7/8-6/8-1/9
=1/8-1/9
=1/72