How to Write an Inverse Function – A Step-by-Step Guide

How to Write an Inverse Function A Step-by-Step Guide

In the realm of mathematics, understanding the concept of an inverse function is akin to learning a secret handshake that reveals a hidden symmetry in the universe of numbers.

When I explore functions, I’m essentially looking at special relationships where every input is paired with exactly one output. Picture a function as a unique dance move that takes me from one point to another, say from point A to point B.

Now, the inverse function is the dance move that takes me back from point B to A, demonstrating a beautiful balance in mathematical operations.

However, not all functions have this mystical partner—only those that are one-to-one, meaning each output is linked to only one input, can boast an inverse.

Knowing whether a function is one-to-one is crucial because, without this property, it cannot have an inverse. To formalize this relationship, we use a particular notation: if ( f(x) ) takes me from A to B, then $ f^{-1}(x)$ is the name of the dance move bringing me back from B to A.

Here’s where understanding the domain and range becomes essential, as the domain of ( f ) becomes the range of $f^{-1}$ and vice versa, giving me a complete picture of this two-way journey.

Steps for Writing Inverse Functions

When I approach the task of finding the inverse function, I think of it as a process of reversing steps to determine how to get back to the original place.

A hand holding a pen writes "f(x)" on the left side of a page, then "y" on the right side. An arrow connects the two, representing the inverse function

Here’s my strategy for problem-solving in algebra and ensuring successful results.

  1. Verify One-to-One: Before defining an inverse function, I ensure that the original function is one-to-one. This means every input has a unique output and vice versa. A handy tool is the horizontal line test: if any horizontal line crosses the graph of the function more than once, it’s not one-to-one and doesn’t have an inverse in its current form.

  2. Switch x and y: I start by replacing the output (usually $y$) with input ($x$) and then solve for $y$. This step swaps the roles of inputs and outputs, preparing the function to be reversed.

  3. Solve for the New Output: Next, I algebraically manipulate the equation to solve for $y$. This often involves function composition, and I take care not to violate the laws of algebra.

  4. Restrict Domains: Some functions, like square roots, need a defined domain to ensure a one-to-one function. Non-negative restrictions are common to avoid multiple results.

  5. Use Proper Notation: It’s vital to denote the inverse function with the correct notation, which is $f^{-1}(x)$. Do not confuse this with reciprocal, as they are not the same.

  6. Double Check Your Work: I perform function composition to confirm that $f(f^{-1}(x)) = x$ and $f^{-1}(f(x)) = x$. If this holds, I’ve found a correct inverse.

Here’s a quick example for clarity:

For the original function $f(x) = 2x + 3$, the steps would look like this:

  1. Since it’s a linear function, it’s inherently one-to-one.
  2. Swap the labels so $x$ becomes $y$ and vice versa, getting $x = 2y + 3$.
  3. I’ll solve for $y$: $y = \frac{x – 3}{2}$.
  4. For this function, I don’t need to restrict the domain.
  5. I apply the notation: $f^{-1}(x) = \frac{x – 3}{2}$.
  6. Verify by composition that $f(f^{-1}(x)) = 2(\frac{x – 3}{2}) + 3 = x$.

By following these steps, I can tackle various inverse trigonometric functions, graphing inverse functions, and even more complex rational functions. Whether it’s a lesson on converting Fahrenheit to Celsius or exploring symmetric graphs, having a solid grasp on inverse functions greatly simplifies these challenges.


In wrapping up our discussion on inverse functions, remember that these special functions are the mathematical equivalent of getting back to where you started.

They are fundamental in many branches of mathematics and real-world applications, like decrypting messages or converting between different units of measurement.

To check whether a function ( f(x) ) has an inverse, I always ensure that it’s a one-to-one function, meaning it passes both the vertical and horizontal line tests.

When I find the inverse of a function, I swap the roles of ( x ) and ( y ) and then solve for the new ( y ), which is the inverse function. Remember to notate the inverse function as $f^{-1}(x)$, which does not mean $(1/f(x))$ but rather the operation that reverses the effect of ( f(x) ).

Keep in mind to represent the inverse graphically as a reflection across the line ( y = x ). Through this representation, the symmetry between a function and its inverse becomes visually apparent and helps in understanding their relationship.

Lastly, I always verify my results. I do this by composing the function and its inverse to see if I get an identity, confirming $f^{-1}(f(x)) = x ) and ( f(f^{-1}(x)) = x $.

This step is crucial to ensure that the inverse I’ve computed undoes what the original function does. With practice, the process of finding and understanding inverse functions will become second nature.