## Precalculus (6th Edition)

See p.407, Horizontal Line Test A function is one-to-one if every horizontal line intersects the graph of the function at most once. ------------- We find many horizontal lines that intersect the graph more than once. One line is enough to fail the test. so we take the x-axis ($y=0)$. Horizontal Line Test: FAIL. The function is not one-to-one.