The three-dimensional Laplace equation
$\mathbb\quad \frac{\partial^{2} f}{\partial x^{2}}+\frac{\partial^{2} f}{\partial y^{2}}+\frac{\partial^{2} f}{\partial z^{2}}=0$
is satisfied by steady-state temperature distributions $T=f(x, y, z)$ in space, by gravitational potentials, and by electrostatic potentials. The two-dimensional Laplace equation
$$
\frac{\partial^{2} f}{\partial x^{2}}+\frac{\partial^{2} f}{\partial y^{2}}=0
$$
obtained by dropping the $\partial^{2} f / \partial z^{2}$ term from the previous equation, describes potentials and steady-state temperature distributions in a plane. The plane may be treated as a thin slice of the solid perpendicular to the $z$ -axis.
Show that each function in Exercises satisfies a Laplace equation.
$$f(x, y)=\tan ^{-1} \frac{x}{y}$$