Theory:

Like we have already dealt with the vertical line test, we will look at the horizontal line test in this section.
 
A horizontal line test is used on a one-one function represented on a graph sheet.
 
Horizontal line test:
A function is said to be one - one if a horizontal line intersects the curve exactly once from any point.
Example:
Let us look at the example given below for a better understanding.
 
10.svg
 
Here, the blue curve denotes the one-one function. The red line indicates the horizontal line drawn for the line test.
 
The red horizontal line intersects the blue curve exactly at one point. Hence, it represents a one-one function.