5) a²+b²/x³+x²y · x²-y²/a⁴-b⁴ ;

$$\frac{a^2+b^2}{x^3+x^2y} \cdot \frac{x^2-y^2}{a^4-b^4} = \frac{a^2+b^2}{x^2(x+y)} \cdot \frac{(x-y)(x+y)}{(a^2-b^2)(a^2+b^2)} = \frac{(x-y)(x+y)}{x^2(x+y)(a^2-b^2)} = \frac{x-y}{x^2(a^2-b^2)}$$

Ответ: $$\frac{x-y}{x^2(a^2-b^2)}$$