$\left\{\begin{matrix}d=\frac{c}{x}\text{, }&x\neq 0\\d\in \mathrm{C}\text{, }&c=0\text{ and }x=0\end{matrix}\right.$

$\left\{\begin{matrix}d=\frac{c}{x}\text{, }&x\neq 0\\d\in \mathrm{R}\text{, }&c=0\text{ and }x=0\end{matrix}\right.$

$c=dx$