In Exercises 15 through 18, show that $u(x, y)$ satisfies the equation
$$
\frac{\partial^{2} u}{\partial x^{2}}+\frac{\partial^{2} u}{\partial y^{2}}=0
$$
which is known as Laplace's equation in $R^{2}$.
$$
u(x, y)=\tan ^{-1} \frac{2 x y}{x^{2}-y^{2}}
$$