Vertical line test
The vertical line test is a way of determining whether or not a plotted graph is a function.
The vertical line test states that a relation is a function iff no vertical line intersects the graph in more than one point.
This is because a function cannot have more than one output for any one input.
For example, is a function because any vertical line intersects it in, at most, one point, while is not a function (try the line ).
In other words, for every x value, there should only be one y value.
This article is a stub. Help us out by expanding it.