5/9+7/12+4/9=(5/9+4/9)+7/12=9/9+7/12=1+7/12=1又7/129/10-（2/5+1/4）=9/10-2/5-1/4=9/10-4/10-1/4=5/10-1/4=1/2-1/4=12/4-1/4=1/47/8-5/12+5/6=7/8-5/12+10/12=7/8+5/12=21/24+10/24=31/244/5+3/10-11/20=4/5+6/2...还有大题目上那道，谢谢7/8+3/4-11/12
=21/24+1/24-22/24
=22/24-22/24
=0