(x-y)的6次方除以（y-x)的3次方除以（x-y)=(x-y)的6次方/[-（x-y)]的3次方/（x-y)，=-(x-y)的6次方/（x-y)的3次方/（x-y),=-(x-y)的(6次方-3次方)/（x-y)，=-(x-y)的3次方/（x-y)，=-(x-y)的(3次方-1次方)，=-(x-y)的2次方,=-(x-y)²
