15. $y=\frac{1}{x}$.
Solution. The denominator becomes zero at $x=0$. The given function takes real values for all $x$, except $x=0$. Therefore, the domain of the given function consists of the intervals $(-\infty, 0)$ and $(0, \infty)$.

$\begin{array}{ll}\text { 16. } y=\frac{2}{1-x} & \text { 17. } y=\frac{3}{x-4}\end{array}$