Which of the following set of points represents a function? (4,3), (2, 1), (2, 9). (9, 4)
(3,5), (4,5), (5,9), (7.-2)
(-1,2). (-1, 4), (-1,8), (-1,9)

Each of the domain values, x, should only output a single unique range value, y.

This example is a relation and not a function because the the domain value of -1 output 2 distinct range values.

-1 outputs a 3

and

-1 outputs a 5

If I were to draw a vertical line at the domain value of -1 that vertical line would intersect the points (-1,3) and (-1,5). Which indicates that the relation is not a function.

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