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The vertical line test allows you to immediately observe whether a graph is a function or not.

How to identify a function in graphs with a vertical line test
FUNCTIONS
1 point
1 point
This image is ONE-to-ONE
Each x-value has one y-value.
No matter the position on the graph, the vertical line only intersects at one point.
This image is MANY-to-ONE
1 point
1 point
Many x-values has one y-value.
No matter the position on the graph, the vertical line only intersects at one point.
NON-FUNCTIONS
This image is ONE-to-MANY
2 points
Each x-values has many y-values. 
No matter the position on the graph, the vertical line intersects at more than one point.
This image is MANY-to-MANY
3 points
 Many x-values has many y-values.
No matter the position on the graph, the vertical line intersects at more than one point.
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