**Identify functions defined by graphs and equations**

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Since each value of *x *in a function corresponds to only one value of *y*, any vertical line drawn through the graph of a function must intersect the graph in at most one point. This is the *vertical line test *for a function.

**Vertical Line Test**

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If every vertical line intersects the graph of a relation in no more than one point, then the relation represents a function.

For example, the graph shown in Figure (a) is not the graph of a function since a vertical line intersects the graph in more than one point. The graph in Figure (b) does represent a function.