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{{ printedBook.courseTrack.name }} {{ printedBook.name }} For a relation to be a function, each $x$-value can only be paired with one $y$-value, but one $y$-value can be paired with multiple $x$-values. In a graph, we can check if each $x$-value has only one $y$-value by subjecting it to the vertical line test.

In the graph, each vertical line passes through the graph at only one point. Therefore, each $x$-value corresponds to one $y$-value and the graph is a function.