Vertical line test

A vertical line test is a visual way to determine if the graph of a relation is a function.
If the vertical line passes through at least two points on the graph, then 1 element in the domain is paired with more than 1 element in the range. 

The line has to be vertical. Below see a correct vertical line test and an incorrect vertical line test.

Next, we show you a few examples where the vertical line test was used to determine if the graph is a function. 

Example #1

Not a functionVertical line test

Notice the two blue points. This means that the vertical line passes through 2 points. 
Can you guess what these two points are just by looking at the graph?

These two points are (4, 1) and (4, 3). As you can see, 4 is paired with two elements and again that is why it is not a function.

Example #2

Is a functionVertical line test

Here, there are 5 vertical lines and none of them passes through at least 2 points. Therefore, this graph is a function.

Feel free to draw as many vertical lines as possible before you draw your conclusion.

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