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 where each input corresponds to just one output. In other words, for each x, there is only one y and vice versa. This implies 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 look at the images below:

One to One Function


For f(x), any value in the left circle corresponds to a unique value in the right circle. In conjunction, any value on the right correlates to a unique value in the left circle. In mathematical jargon, this means that every domain holds a unique range, and every range has a unique domain. Hence, this is an example of a one-to-one function.

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

  • f(x) = x + 1

  • f(x) = 2x

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

Pay attention to the fact that the inputs in the left circle (domain) do not have unique outputs in the right circle (range). Case in point, 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 equivalent Y values for many X values. Thus, 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 qualities of One to One Functions?

One-to-one functions have these properties:

  • The function holds 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 regarding the line y = x.

How to Graph a One to One Function

When trying to graph a one-to-one function, you are required to figure out the domain and range for the function. Let's study a straight-forward example of a function f(x) = x + 1.

Domain Range

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

How can you tell whether a Function is One to One?

To prove if a function is one-to-one, we can use the horizontal line test. Once you chart the graph of a function, trace horizontal lines over the graph. If a horizontal line moves through the graph of the function at more than one spot, then the function is not one-to-one.

Due to the fact that 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 reason that all linear functions are one-to-one functions. Remember that we do not apply the vertical line test for one-to-one functions.

Let's examine the graph for f(x) = x + 1. Immediately after you chart the values for the x-coordinates and y-coordinates, you need to examine whether a horizontal line intersects the graph at more than one point. In this example, the graph does not intersect any horizontal line more than once. This means 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 examine the figure 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 instance, the graph intersects numerous horizontal lines. Case in point, for each domains -1 and 1, the range is 1. Additionally, for either -2 and 2, the range is 4. This signifies that f(x) = x^2 is not a one-to-one function.

What is the inverse 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 is also a one-to-one function. The inverse of the function basically reverses the function.

For example, in the example of f(x) = x + 1, we add 1 to each value of x in order to get the output, i.e., y. The opposite of this function will deduct 1 from each value of y.

The inverse of the function is denoted as f−1.

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

The characteristics of an inverse one-to-one function are identical to any other one-to-one functions. This means that the opposite of a one-to-one function will possess one domain for each range and pass the horizontal line test.

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

Finding the inverse of a function is simple. You simply have 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 previously, the inverse of a one-to-one function reverses the function. Since the original output value required us 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

Contemplate these 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 any of these functions:

1. Determine whether or not the function is one-to-one.

2. Graph the function and its inverse.

3. Determine the inverse of the function numerically.

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

5. Apply the inverse to find the solution for x in each formula.

Grade Potential Can Help You Learn You Functions

If you are struggling trying to learn one-to-one functions or similar topics, Grade Potential can put you in contact with a one on one tutor who can assist you. Our Youngstown math tutors are experienced professionals who help students just like you advance their understanding of these concepts.

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