The vertical line test is a method that is used to determine whether a given relation is a function or not. The approach is rather simple. Draw a vertical line cutting through the graph of the relation, and then observe the points of intersection.
The vertical line test supports the definition of a function. That is, every x-value of a function must be paired to a single yy-value. If we think of a vertical line as an infinite set of x-values, then intersecting the graph of a relation at exactly one point by a vertical line implies that a single x-value is only paired to a unique value of y.
If a vertical line intersects the graph in all places at exactly one point, then the relation is a function.
If a vertical line intersects the graph in some places at more than one point, then the relation is NOT a function.
With the above illustration,
Graph 3 is not a function using vertical test because it has infinitely many solutions
Graph 4 is not a function because the vertical cuts the graph and intersect at a points
Graph 5 is a constant function because it has a repeated value of x and a constant y value
Graph 6 is not a function because the x-values which is the domain is repeated for different values of y
Graph 7 is not a function because the vertical line cuts the graph at two-point
Graph 8 is a function because it has one point intersection and each value of x have a corresponding value of y