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eg3382
it is a visual way to determine if a curve is a graph of a function or not. a function can only have one output, y, for each unique input, x. 
TheBlueFox

The vertical line test is very useful for determining if a relation represents a function if it’s on a graph. The vertical line test works like this. If we can draw a vertical line that passes through more than one point on the graph, then the relation is not a function.