## Precalculus (6th Edition) Blitzer

An equation involving the variables $x$ and $y$ is a function if, after solving for $y$ in terms of $x$, every value of $x$ gives only one value of $y$.
An equation involving the variables $x$ and $y$ is a function if, after solving for $y$ in terms of $x$, every value of $x$ gives only one value of $y$.