May 27, 2022

One to One Functions - Graph, Examples | Horizontal Line Test

What is a One to One Function?

A one-to-one function is a mathematical function in which each input correlates to a single output. That is to say, for each x, there is a single y and vice versa. This signifies that the graph of a one-to-one function will never intersect.

The input value in a one-to-one function is noted as the domain of the function, and the output value is noted as the range of the function.

Let's study the images below:

One to One Function


For f(x), any value in the left circle correlates to a unique value in the right circle. In the same manner, any value in the right circle corresponds to a unique value on the left. In mathematical terms, this signifies every domain owns a unique range, and every range holds a unique domain. Hence, this is an example of a one-to-one function.

Here are some more representations of one-to-one functions:

  • f(x) = x + 1

  • f(x) = 2x

Now let's look at the second example, which displays the values for g(x).

Be aware of the fact that the inputs in the left circle (domain) do not hold unique outputs in the right circle (range). Case in point, the inputs -2 and 2 have identical output, i.e., 4. Similarly, the inputs -4 and 4 have equal output, i.e., 16. We can discern that there are identical Y values for multiple X values. Hence, this is not a one-to-one function.

Here are different representations of non one-to-one functions:

  • f(x) = x^2

  • f(x)=(x+2)^2

What are the qualities of One to One Functions?

One-to-one functions have the following qualities:

  • The function has an inverse.

  • The graph of the function is a line that does not intersect itself.

  • It passes the horizontal line test.

  • The graph of a function and its inverse are identical with respect to the line y = x.

How to Graph a One to One Function

In order to graph a one-to-one function, you will have to find the domain and range for the function. Let's look at an easy example of a function f(x) = x + 1.

Domain Range

Once you have the domain and the range for the function, you ought to plot the domain values on the X-axis and range values on the Y-axis.

How can you evaluate whether or not a Function is One to One?

To prove whether or not a function is one-to-one, we can use the horizontal line test. Immediately after you graph the graph of a function, trace horizontal lines over the graph. If a horizontal line passes through the graph of the function at more than one place, then the function is not one-to-one.

Because the graph of every linear function is a straight line, and a horizontal line does not intersect the graph at more than one spot, we can also deduct all linear functions are one-to-one functions. Keep in mind that we do not apply the vertical line test for one-to-one functions.

Let's look at the graph for f(x) = x + 1. As soon as you chart the values of x-coordinates and y-coordinates, you have to review whether or not a horizontal line intersects the graph at more than one spot. In this case, the graph does not intersect any horizontal line more than once. This signifies that the function is a one-to-one function.

On the contrary, if the function is not a one-to-one function, it will intersect the same horizontal line more than one time. Let's look at the diagram for the f(y) = y^2. Here are the domain and the range values for the function:

Here is the graph for the function:

In this case, the graph intersects multiple horizontal lines. Case in point, for both domains -1 and 1, the range is 1. Additionally, for either -2 and 2, the range is 4. This implies that f(x) = x^2 is not a one-to-one function.

What is the opposite of a One-to-One Function?

As a one-to-one function has just one input value for each output value, the inverse of a one-to-one function is also a one-to-one function. The opposite of the function basically reverses the function.

Case in point, in the case of f(x) = x + 1, we add 1 to each value of x for the purpose of getting the output, in other words, y. The inverse of this function will deduct 1 from each value of y.

The inverse of the function is f−1.

What are the qualities of the inverse of a One to One Function?

The qualities of an inverse one-to-one function are the same as any other one-to-one functions. This signifies that the reverse of a one-to-one function will possess one domain for every range and pass the horizontal line test.

How do you determine the inverse of a One-to-One Function?

Figuring out the inverse of a function is simple. You just need to swap the x and y values. Case in point, the inverse of the function f(x) = x + 5 is f-1(x) = x - 5.


Just like we learned previously, the inverse of a one-to-one function reverses the function. Considering the original output value required adding 5 to each input value, the new output value will require us to delete 5 from each input value.

One to One Function Practice Questions

Examine the subsequent functions:

  • f(x) = x + 1

  • f(x) = 2x

  • f(x) = x2

  • f(x) = 3x - 2

  • f(x) = |x|

  • g(x) = 2x + 1

  • h(x) = x/2 - 1

  • j(x) = √x

  • k(x) = (x + 2)/(x - 2)

  • l(x) = 3√x

  • m(x) = 5 - x

For each of these functions:

1. Identify whether the function is one-to-one.

2. Draw the function and its inverse.

3. Find the inverse of the function numerically.

4. State the domain and range of each function and its inverse.

5. Employ the inverse to find the solution for x in each equation.

Grade Potential Can Help You Learn You Functions

If you are facing difficulties trying to understand one-to-one functions or similar functions, Grade Potential can set you up with a one on one tutor who can support you. Our Anaheim math tutors are skilled educators who help students just like you advance their mastery of these types of functions.

With Grade Potential, you can learn at your own pace from the comfort of your own home. Schedule a meeting with Grade Potential today by calling (714) 410-2473 to learn more about our teaching services. One of our team members will contact you to better ask about your needs to provide you with the best tutor for you!

Let Grade Potential set you up with the perfect Grammar tutor!

Or answer a few questions below to get started