Given function .

Horizontal line test:

A function is one-to-one if and only if no horizontal line intersects its graph more than once.

That is, any horizontal line in the graph must cross the curve at one point only.

Graph :

 Graph  the function .

Draw the horizontal line .

Observe the graph:

The horizontal line touches the graph of the function more than  one point.

The function is  not an one-to-one, because it does not satisfy the horizontal line test.

The function is  not an one-to-one, because it does not satisfy the horizontal line test.