Vertical line test

A vertical line test is a visual way to determine if the graph of a relation is a function.
If the vertical line passes through at least two points on the graph, then 1 element in the domain is paired with more than 1 element in the range. 

The line has to be vertical. Below see a correct vertical line test and an incorrect vertical line test.

Next, we show you a few examples where the vertical line test was used to determine if the graph is a function. 

Example #1

Not a functionVertical line test

Notice the two blue points. This means that the vertical line passes through 2 points. 
Can you guess what these two points are just by looking at the graph?

These two points are (4, 1) and (4, 3). As you can see, 4 is paired with two elements and again that is why it is not a function.

Example #2

Is a functionVertical line test

Here, there are 5 vertical lines and none of them passes through at least 2 points. Therefore, this graph is a function.

Feel free to draw as many vertical lines as possible before you draw your conclusion.




Recent Articles

  1. How to Derive the Equation of an Ellipse Centered at the Origin

    Mar 13, 19 11:50 AM

    Learn how to derive the equation of an ellipse when the center of the ellipse is at the origin.

    Read More

New math lessons

Your email is safe with us. We will only use it to inform you about new math lessons.

            Follow me on Pinterest


Math quizzes

 Recommended

Scientific Notation Quiz

Graphing Slope Quiz

Adding and Subtracting Matrices Quiz  

Factoring Trinomials Quiz 

Solving Absolute Value Equations Quiz  

Order of Operations Quiz

Types of angles quiz


Tough algebra word problems

Tough Algebra Word Problems.

If you can solve these problems with no help, you must be a genius!

Recent Articles

  1. How to Derive the Equation of an Ellipse Centered at the Origin

    Mar 13, 19 11:50 AM

    Learn how to derive the equation of an ellipse when the center of the ellipse is at the origin.

    Read More

K-12 math tests


Everything you need to prepare for an important exam!

K-12 tests, GED math test, basic math tests, geometry tests, algebra tests. 

Real Life Math Skills

Learn about investing money, budgeting your money, paying taxes, mortgage loans, and even the math involved in playing baseball.