The function is .

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  in .

Draw the horizontal line .

Observe the graph:

The horizontal line  touches the graph of the function exactly at one point.

The function is an one-to-one, because it passes the horizontal line test.

The function is an one-to-one function.