先(—1/30)等于—1/30,后面的把它全部通分成(20/30—3/30+5/30—12/30)等于10/30,即1/3,最后把—1/30乘以1/3等于—1/90。所以最后等于—1/90
-1/30 / (20/30-3/10+5/30-12/30)
=-1/30 /(10/30)
=-1/30*3
=-1/10
由1/30与2/3,1/10 1/6,2/5知3*10=5*6=30则
=(-1/30)/(17/30-7/30)
30抵消
得-1*0.1=-0.1
=(-1/30)/[(2/3+1/6)-(1/10+2/5)]
=(-1/30)/[(4/6+1/6)-(1/10+4/10)]
=(-1/30)/(5/6-5/10]
=(-1/30)/(5/6-3/6)
=(-1/30)/(2/6
=-3/10