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 corresponds to just one output. In other words, for every x, there is just one 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 known 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, each value on the right correlates to a unique value in the left circle. In mathematical words, this implies every domain owns a unique range, and every range owns a unique domain. Thus, this is an example of a one-to-one function.

Here are some different examples of one-to-one functions:

  • f(x) = x + 1

  • f(x) = 2x

Now let's examine the second picture, which exhibits the values for g(x).

Notice that the inputs in the left circle (domain) do not own unique outputs in the right circle (range). For example, the inputs -2 and 2 have the same output, that is, 4. Similarly, the inputs -4 and 4 have the same output, i.e., 16. We can discern that there are matching Y values for many X values. Therefore, this is not a one-to-one function.

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

  • f(x) = x^2

  • f(x)=(x+2)^2

What are the characteristics of One to One Functions?

One-to-one functions have the following properties:

  • The function owns 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 are required to determine the domain and range for the function. Let's look at an easy example of a function f(x) = x + 1.

Domain Range

As soon as you possess 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 a function is one-to-one, we can use the horizontal line test. Immediately after you plot the graph of a function, draw horizontal lines over the graph. If a horizontal line moves 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 place, we can also reason that all linear functions are one-to-one functions. Don’t forget that we do not use the vertical line test for one-to-one functions.

Let's study the graph for f(x) = x + 1. Once you chart the values for the x-coordinates and y-coordinates, you have to review whether a horizontal line intersects the graph at more than one place. In this example, the graph does not intersect any horizontal line more than once. This signifies that the function is a one-to-one function.

On the other hand, if the function is not a one-to-one function, it will intersect the same horizontal line multiple times. Let's look at the graph 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 example, the graph crosses various horizontal lines. For instance, for either domains -1 and 1, the range is 1. In the same manner, for both -2 and 2, the range is 4. This signifies that f(x) = x^2 is not a one-to-one function.

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

Considering the fact that a one-to-one function has just one input value for each output value, the inverse of a one-to-one function also happens to be a one-to-one function. The opposite of the function basically reverses the function.

For Instance, in the event 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 opposite of this function will deduct 1 from each value of y.

The inverse of the function is f−1.

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

The qualities of an inverse one-to-one function are no different than all other one-to-one functions. This implies that the reverse of a one-to-one function will hold one domain for each range and pass the horizontal line test.

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

Determining the inverse of a function is not difficult. You simply 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.


As we learned earlier, the inverse of a one-to-one function reverses the function. Since the original output value showed us we needed to add 5 to each input value, the new output value will require us to subtract 5 from each input value.

One to One Function Practice Examples

Consider the following 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. Determine if the function is one-to-one.

2. Graph the function and its inverse.

3. Figure out the inverse of the function numerically.

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

5. Employ the inverse to solve for x in each calculation.

Grade Potential Can Help You Master You Functions

If you find yourself struggling trying to understand one-to-one functions or similar concepts, Grade Potential can connect you with a private tutor who can assist you. Our Clear Water math tutors are skilled educators who assist students just like you improve their mastery of these types of functions.

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