(1)ab分之a+b - bc分之bc
(a+b)/ab -(b+c)/bc
=[(a+b)c-(b+c)a]/abc
=(ac+bc-ab-ac)/abc
=(bc-ab)/abc
=b(c-a)/abc
=(c-a)/ac
(2)(x-3)的平方分之3x - 3-x分之x
=3x/(x-3)² - x/(3-x)
=3x/(x-3)² + x/(x-3)
=3x/(x-3)² + x(x-3)/(x-3)²
=(3x+x²-3x)/(x-3)²
=x²/(x-3)²