18. \( y=\frac{1}{2 x-5} \).
Solution. Here $2 x-5 \neq 0$, hence $x \neq 2.5$. Thus, we get the answer: $(-\infty ; 2.5)$ and $(2.5 ; \infty)$.

$\begin{array}{ll}\text { 19. } y=\frac{x-1}{x+1} & \text { 20. } y=\frac{2 x+1}{3 x-1} \text {. }\end{array}$