Решение:

a) $$\begin{aligned} (\frac{x+y}{x-y} - \frac{x-y}{x+y}) : \frac{xy}{x^2-y^2} &= \frac{(x+y)^2 - (x-y)^2}{(x-y)(x+y)} : \frac{xy}{x^2-y^2} = \frac{x^2 + 2xy + y^2 - (x^2 - 2xy + y^2)}{(x-y)(x+y)} : \frac{xy}{x^2-y^2} = \frac{4xy}{(x-y)(x+y)} \cdot \frac{x^2-y^2}{xy} = \frac{4xy}{(x-y)(x+y)} \cdot \frac{(x-y)(x+y)}{xy} = 4 \end{aligned}$$

Ответ: 4