图
全部通分
就是直接通分就行。7/8 - 3/4 + 1/2= 7/8 - 6/8 + 4/8= (7 - 3 + 4)/8= 8/8= 1
7/8 - 6/8 + 4/8 = (7-6+4) / 8 = 5/8
=7/8-6/8+4/8=5/8
