4) x-y.x2-y2 :. xy 3xy

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

Ответ: $$\frac{3}{x+y}$$