Hey!
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In a function ONE input-value can be assigned to only ONE output-value.
We can use the vertical line test to see whether each graph is a function or not.
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Graph 1, 5, and 6 are functions. When we draw a line vertically we can see that the line only passes through 1 point on the graph.
Graph 2, 3, and 4 are NOT functions. When we draw a line vertically we can see that the line passes through more then 1 point on the graph.
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Hope This Helped! Good Luck!