<span>This characteristic of non-functions was noticed by I-don't-know-who, and was codified in "The Vertical Line Test": Given the graph of a relation, if you can draw a vertical line that crosses the graph in more than one place, then the relation is not a function.</span>
Also, if you can get all the coordinates----> (3,5) 3=x value 5=y value
If the x value ever appears more than once then it's not a function Example (1,5) (2,6) (3,9) (2,4) Is that a function? No, because the x value, 2 appears twice.
Example: (4,5) (2,3) (1,5) (3,7) Is that a function? Yes, because non of the x values appear more than once, there for the vertical line test will nvr have 2 intersections
