A function of two variables $z=f(x, y)$ maps each ordered pair $(x, y)$ in a subset $D$ of the real plane $\mathbb{R}^{2}$ to a unique real number $z$. The set $D$ is called the domain of the function. The range of $f$ is the set of all real numbers $z$ that has at least one ordered pair $(x, y) \in D$ such that $f(x, y)=z$ as shown in the following figure.