662. a) \(\frac{5(x-y)}{15(y-x)}\);

$$\frac{5(x-y)}{15(y-x)} = \frac{5(x-y)}{-15(x-y)} = \frac{5}{-15} \cdot \frac{(x-y)}{(x-y)} = -\frac{1}{3} \cdot 1 = -\frac{1}{3}$$ Ответ: \(-\frac{1}{3}\)