Question 568670: How do you tell on a coordinate graph if it is a function or not????
Answer by richard1234(7193)   (Show Source):
You can put this solution on YOUR website! If there are two points on the graph of the function that have the same x-coordinate, but different y-coordinates, the relation cannot be a function (assuming x is the independent variable, one x cannot map to two or more y). You can check this by (mentally) drawing vertical lines on the coordinate plane. If any of the lines touches the graph at two points, the relation cannot be a function. This is usually referred to as the "vertical line test."
If the vertical line test passes (no vertical lines touch at 2+ points) then the relation is a function, even if it cannot easily be defined.
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