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Two ways to determine whether the relation is a function is use a mapping diagram or use a vertical line test.
The vertical line test: Imagine a very large family of vertical lines. If any of the lines intersect with the graph of the relation under consideration at more than a single point then the relation is not a function. (Because a function assigns just one value in the range to a given point in the domain.)
You can use the vertical line test to determine if a relation is a function. It's pretty simple: if there is any part of the graph where there are more than one of the same x-values for different y-values (ex. (3,2), (3,5), and (3,9)), the relation is not a function
A function can only have one output for any given input. This means that any x value you choose cannot have multiple corresponding y values. The vertical line test involves looking at a graph and drawing vertical lines over it. If any of the vertical lines you have drawn touch the graph of the function more than once, then the graph does not represent a function.
This graph fails the vertical line test at x = 3This graph is not the graph of a function.