## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

The equation defines $y$ is a function of $x$.
For an equation to be a function, it must satisfy the following requirement: Every value of $x$ must correspond to exactly one value of $y$. In this case it is true, because if we plug in any number in the place of $x$ the value of $y$ will be clear. For example, for $x=2$, $\frac{1}{x}=\frac{1}{2}=0.5$, therefore $y$ can only have $0.5$ as a value if $x=2$.