Functions cannot have two y-values (outputs) for any single x-value (input), so if you can draw a vertical line that touches more than 1 point on the graph, it is not a function.
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The "vertical line test" will tell you if it is a function or not. The graph is not a function if it is possible to draw a vertical line through two points.
If a vertical line, within the domain of the function, intersects the graph in more than one points, it is not a function.
You use the vertical line test. If you can draw a vertical line though the graph and it intersects it only once, it is a function. If the line crosses the graphs more than once it is not.
Draw a vertical line if the line hits more than one point on the graph then it is not a function.
A graph can represent either a relation or a function, depending on the nature of the relationship between the variables depicted. A relation is simply a set of ordered pairs, while a function is a specific type of relation where each input (or x-value) is associated with exactly one output (or y-value). To determine if a graph represents a function, the vertical line test can be applied: if any vertical line intersects the graph at more than one point, it is not a function.