1-2/5+3/7=3/5+3/7=3×(1/5+1/7)=3×12/35=36/35 7/衡斗薯10-(1/2-1/3) =7/10-1/2+1/3=1/5-1/3=2/1511/12-(3/4-7/咐者30) =11/12-3/销嫌4+7/30=1/6+7/30=2/5