解法一：ob=ab-ao=ab-1/2ac=ab-1/2(ab+bc)=1/2(ab-bc)=1/2(4-3)=0.5
解法二：ob=oc-bc=1/2ac-bc=1/2(ab+bc)-ac=1/2(ab-bc)=1/2(4-3)=0.5