a) Mẫu thức chung: $3(x-4)(x+4)$.
Khi đó
$\dfrac{1}{3x-12} = \dfrac{1}{3(x-4)} = \dfrac{x+4}{3(x-4)(x+4)}$
$\dfrac{2}{3x+12} = \dfrac{2}{3(x+4)} = \dfrac{2(x-4)}{3(x-4)(x+4)}$
$\dfrac{3}{16-x^2} = -\dfrac{3}{(x-4)(x+4)} = -\dfrac{9}{3(x-4)(x+4)$.
b) Mẫu thức chung: $(x-2)(x+2)$
Khi đó
$\dfrac{x^2-4}{x^2-4x+4} = \dfrac{(x-2)(x+2)}{(x-2)^2}$
$= \dfrac{x+2}{x-2}$
$= \dfrac{(x+2)^2}{(x-2)(x+2)}$
$= \dfrac{x^2 + 4x + 4}{(x-2)(x+2)}$
và
$\dfrac{2x}{x^2-4} = \dfrac{2x}{(x-2)(x+2)}$
