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When there is one and only one y-value for each x-value (no duplicates in y), it is called a one-to-one
function.
You can determine if a relation is a function by looking at a table of values.
Examples
You can also determine if a relation is a function from its graph using the vertical-line test. The vertical
line test can be done with any straight object, like a pencil. Pass the pencil vertically across the graph, and
if it crosses the line of the relation more than once, it is not a function. (By the way, a one-to-one function
also passes the horizontal-line test.)
