有小括号时
(3/4+?)*2/3-1/8
=1/2+?2/3-1/8
=3/8+?2/3
无括号
3/4+?2/3-1/8
=5/8+?2/3
两者相差:
5/8+?2/3-(3/8+?2/3)
=5/8-3/8
=1/4