Identify functions defined by graphs and equations
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
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.