Problem 18 - College Algebra

Question

The functions are all one-to-one. For each function, a. Find an equation for \(f^{-1}(x),\) the inverse function. b. Verify that your equation is correct by showing that $$f\left(f^{-1}(x)\right)=x \text { and } f^{-1}(f(x))=x$$ $$ f(x)=x^{3}-1 $$

Step-by-Step Solution

Answer

The inverse function of \(f(x) = x^3 - 1\) is \(f^{-1}(x) = \sqrt[3]{x+1}\). Verification of \(f(f^{-1}(x)) = x\) and \(f^{-1}(f(x)) = x\) confirms this result.

Step 1: Finding the inverse function

Firstly, an inverse function of \(f(x)\) is obtained by swapping \(x\) and \(f(x)\) in the original function and resolving for \(x\). So for function \(f(x) = x^3 - 1\), after switching \(x\) and \(f(x)\) we get \(f^{-1}(x) = x^3 - 1\). Solving this for \(x\) gives us \(f^{-1}(x) = \sqrt[3]{x+1}\).

Step 2: Verifying the inverse function

Next, it is necessary to prove that \(f(f^{-1}(x)) = x\) and \(f^{-1}(f(x)) = x\). Starting with \(f(f^{-1}(x))\), substitute \(f^{-1}(x)\) from Step 1 into \(f(x)\) to get \(f(\sqrt[3]{x+1}) = (\sqrt[3]{x+1})^3 - 1 = x - 1 + 1 = x\). For \(f^{-1}(f(x))\), substitute \(f(x)\) from the original function into \(f^{-1}(x)\) from Step 1 to get \(f^{-1}(x^3 - 1) = \sqrt[3]{(x^3 - 1) + 1} = \sqrt[3]{x^3} = x\).

Step 3: Interpreting the results

Given that \(f(f^{-1}(x)) = x\) and \(f^{-1}(f(x)) = x\), it can be concluded that the inverse function found is correct. This confirms that \(f(x)\) and \(f^{-1}(x)\) are indeed inverse functions.

Key Concepts

Finding Inverse Functions

One of the most intriguing aspects of mathematical functions is the concept of finding inverse functions. The idea is to map from the range back to the domain in such a way that it 'undoes' the original function.

First and foremost, to find the inverse of a function, you replace the function notation, typically expressed as \(f(x)\), with \(y\). Then you switch the roles of \(x\) and \(y\), and solve the new equation for \(y\). This new \(y\) is your inverse function, often denoted as \(f^{-1}(x)\). For instance, with the given function \(f(x) = x^3 - 1\), you would rewrite it as \(y = x^3 - 1\), interchange \(x\) and \(y\) to get \(x = y^3 - 1\), and then solve for \(y\), resulting in \(f^{-1}(x) = \sqrt[3]{x+1}\).

Key Steps in Finding Inverse Functions

• Replace \(f(x)\) with \(y\).
• Interchange \(x\) and \(y\) in the equation.
• Solve the resulting equation for \(y\).
• Express \(y\) in terms of \(x\) to find \(f^{-1}(x)\).

One-to-One Functions

An essential prerequisite for a function to have an inverse is that it must be one-to-one. A one-to-one function, or injective function, is defined by its property that no two different elements in the domain map to the same element in the range.

In other words, each input has a unique output. This characteristic ensures that when you find an inverse, every output from the original function corresponds to one, and only one, input from the inverse function. For verification purposes, a common method to test if a function is one-to-one is to use the horizontal line test, which states that if any horizontal line intersects the function's graph at most once, the function is one-to-one.

Why Must Functions be One-to-One to Have an Inverse?

Because if there were two different inputs with the same output, you couldn't decide which one to pick when going backwards from the range to the domain. The uniqueness of mapping is what allows an inverse to exist.

Verifying Inverse Functions

Simply finding an expression for the inverse function does not guarantee its correctness. Verification is a crucial step to ensure that the inverse truly represents the original function 'in reverse.'

To verify an inverse function, you must demonstrate that composing the original function with its inverse yields the identity function. Mathematically, this is shown by proving that \(f(f^{-1}(x)) = x\) and \(f^{-1}(f(x)) = x\) for all values in the domain and range of the functions, respectively. Through substitution and appropriate simplifications, proving these two equations confirm that the functions are inverses of each other.

Steps to Verify an Inverse Function

• Substitute the inverse function into the original function and simplify. You should arrive at \(x\).
• Substitute the original function into the proposed inverse function and simplify. Again, you should arrive at \(x\).
• If both conditions are met, the functions are inverses of each other.

Following these methods not only proves the inverse function's validity but reinforces the understanding of the interdependent nature of functions and their inverses.