Solve u=arctany/x

$$u = \arctan \frac { y } { x }$$

Solve for u

$$u=\arctan(\frac{y}{x})$$

$$x\neq 0$$

Solve for x

$$\left\{\begin{matrix}x=y\cot(u)\text{, }&\exists n_{2}\in \mathrm{Z}\text{ : }\left(u>\frac{\pi n_{2}}{2}\text{ and }u<\frac{\pi n_{2}}{2}+\frac{\pi }{2}\right)\text{, }not(n_{2}<-1)\text{ and }not(n_{2}>0)\text{ and }y\neq 0\\x\neq 0\text{, }&\exists n_{1}\in \mathrm{Z}\text{ : }u=\pi n_{1}\text{, }n_{1}=0\text{ and }y=0\end{matrix}\right.$$