Bài 8. Quy đồng: a, \( \frac{1}{xy} \); \( \frac{1}{x^2-y^2} \); \( \frac{1}{x^3-y^3} \) b, \( \frac{1}{a^2-4} \); \( \frac{1}{a^3-8} \); \( \frac{1}{a+2} \)