A vertical line can be used to test whether or not a graph is 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.
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.
A vertical line!
A vertical line has an undefined slope. For the line to be parallel to a vertical line, the slopes would have to be the same. Therefore, the line parallel to a vertical line also has an undefined slope.
A horizontal line is a line perpendicular to the vertical.