7. Пусть \(x-y=3\) и \(z=-5\). Найдите: 1) \(x-y+z\); 3) \(x-2z-y\); 5) \(\frac{z}{x-y-z}\).

1. \(x-y+z=3+(-5)=-2\).
2. \(x-2z-y=(x-y)-2z=3-2\cdot(-5)=3+10=13\).
3. \(\frac{z}{x-y-z}=\frac{-5}{3-(-5)}=\frac{-5}{8}\).

Ответ: 1) −2; 3) 13; 5) \(-\frac58\).