x(x的2次方+xy+y的2次方)-y(x的2次方+xy+y的2次方)+3xy(y-x)=(x-y)(x的2次方+xy+y的2次方)+3xy(y-x)=(x-y)(x的2次方-2xy+y的2次方)=(x-y)的3次方=-8分之1
