Problem 24 - 5 Steps to A 5: AP Calculus AB 2021

Question

Given \(g(x)=3 x-12,\) find \(g^{-1}(3)\).

Step-by-Step Solution

Answer

Answer: \(g^{-1}(3) = 5\).

Step 1: Write down the given function

We have the function \(g(x)=3x-12\).

Step 2: Replace \(g(x)\) with \(y\)

Let \(y=3x-12\).

Step 3: Swap \(x\) and \(y\)

Now, we swap \(x\) and \(y\). This gives us \(x=3y-12\).

Step 4: Solve for \(y\)

To solve for \(y\), we follow these steps: 1. Add \(12\) to both sides: \(x+12=3y\). 2. Divide both sides by \(3\): \(y=\frac{x+12}{3}\).

Step 5: Replace \(y\) with \(g^{-1}(x)\)

Now, we replace \(y\) with \(g^{-1}(x)\) to get the inverse function: \(g^{-1}(x)=\frac{x+12}{3}\).

Step 6: Substitute the value \(3\) into the inverse function

Finally, we substitute the given value \(3\) into the resulting inverse function to find \(g^{-1}(3)\): $$g^{-1}(3)=\frac{3+12}{3}=\frac{15}{3}=5$$. So, \(g^{-1}(3) = 5\).

Key Concepts

Function Inversion

Function inversion is a process used to find an inverse function, essentially reversing the roles of dependent and independent variables. If you have a function, the inverse will unravel the process or steps applied in the original function. To understand this concept, consider a simple example. If the function describes how you determine your journey time given speed, the inverse would describe how to figure out speed given the journey time.

• Steps to Find an Inverse Function:
• Start by replacing the function notation, say, \( g(x) \) with \( y \) to make manipulation easier.
• Then, interchange the roles of \( x \) and \( y \), flipping the function around.
• Finally, solve the equation for \( y \). This resulting expression in terms of \( x \) is your inverse function.

Applying these steps to the example given above for \( g(x) = 3x - 12 \), we replace \( g(x) \) with \( y \), swap \( x \) and \( y \), and solve for \( y \) to get the inverse. The operation of inversion is critical when you need to go backwards or reverse a process.

Linear Functions

Linear functions are foundational in algebra, characterized by expressions like \( y = mx + b \). Here, \( m \) is the slope, and \( b \) is the y-intercept. Linear functions graph as straight lines. They are simple yet powerful tools for many real-life scenarios, from business forecasts to physics calculations. Understanding linear functions helps in grasping more complex mathematical concepts later on.

• Properties of Linear Functions:
• The graph is a straight line, which makes predictions simple and straightforward.
• The slope \( m \) represents the rate of change; a positive slope rises as it moves right, while a negative slope falls.
• The y-intercept \( b \) is the point where the line crosses the y-axis.

In the given exercise, \( g(x) = 3x - 12 \) is a linear function. Here, 3 stands for the slope, indicating that for each unit increase in \( x \), \( y \) increases by 3. The \(-12\) indicates the initial value when \( x = 0 \). Recognizing the form of linear functions is crucial for algebraic manipulation and understanding their real-world applications.

Algebraic Manipulation

Algebraic manipulation involves rearranging equations to isolate a particular variable. This skill is essential when solving for unknowns, simplifying expressions, or finding inverses. It is a core part of algebra and necessary for making sense of more advanced topics. When manipulating expressions, performing operations consistently on both sides of an equation is key to maintaining equality.

• Steps in Algebraic Manipulation:
• Add or subtract terms from both sides to move variables and constants.
• Use multiplication or division to isolate the variable of interest.
• Simplify the resulting expression, if necessary.

In the provided solution, after swapping \( x \) and \( y \), the equation \( x = 3y - 12 \) is manipulated algebraically to solve for \( y \). Adding 12 and then dividing by 3 efficiently isolates \( y \), leading to the inverse function \( g^{-1}(x) = \frac{x + 12}{3} \). Mastery of algebraic manipulation will boost your confidence in untangling equations and finding solutions to mathematical problems.