a-b=2,a-c=1/2,则b-c=-3/2，所以(-3/2)的3次方-3(-3/2)+9/4=27/8
b-c=(a-c)-(a-b)=-3/2 (b-c)的3次方-3(b-c)+9/4=-27/8 +9/2+9/4=-27/8+27/4=27/8
