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

$\text{not a function}$
A relation is a function if each element in the first set is associated with exactly one element in the second set. This relation isn't a function as there are two ordered pairs, $(-4,4)$ and $(-4,0)$, that have the same first element but different second elements.