10) \(\dfrac{5(x-y)}{a(y-x)}=\)

Поскольку \(y-x=-(x-y)\), то:

\(\dfrac{5(x-y)}{a(y-x)}=\dfrac{5(x-y)}{-a(x-y)}=-\dfrac{5}{a}\).

Ответ: \(-\dfrac{5}{a}\), при \(a\ne0\) и \(x\ne y\).