1/2+(-2/3)+4/5+(-1/2)+(-1/3)=[1/2+(-1/2)]+4/5+[(-1/3)+(-2/3)]=0+4/5+-1-1/5成功!!!!!!!!!!!!!!!!!!!!加油!!!!!!!!!!!!!!!!!!!!
1/2+(-2/3)+4/5+(-1/2)+(-1/3)=1/2-(1/2)-[2/3+)+(1/3)]+4/5=4/5-1=-1/5
-1/5
