663. a) \(\frac{2a(x+y)}{8a(x+y)(x-y)}\);

$$\frac{2a(x+y)}{8a(x+y)(x-y)} = \frac{2a}{8a} \cdot \frac{(x+y)}{(x+y)} \cdot \frac{1}{(x-y)} = \frac{1}{4} \cdot 1 \cdot \frac{1}{(x-y)} = \frac{1}{4(x-y)}$$ Ответ: \(\frac{1}{4(x-y)}\)