Horizontal Line Test
A test use to determine if a function is one-to-one.
If a horizontal line intersects
a function's graph more than once, then the function is not one-to-one.
Note:
The function y = f(x) is a
function if it passes the vertical
line test. It is a one-to-one
function if it
passes both the vertical line test and the horizontal line test.

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