SOLUTION: How can I tell if a graph is a function or not?

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Question 706938: How can I tell if a graph is a function or not?
Answer by Edwin McCravy(20063) About Me  (Show Source):
You can put this solution on YOUR website!
All these amount to the same thing:

1. There can be no vertical line that crosses the graph of a function twice.
2. The graph of a function cannot have two points with the same x-coordinate.
3. The graph of a function cannot pass above itself.
4. The graph of a function cannot pass below itself.
5. The graph of a function cannot have two points one directly above the other.