Question

Ron and Christine are determining the composite function \((f \circ g)(x),\) where \(f(x)=x^{2}+x-6\) and \(g(x)=x^{2}+2 .\) Who is correct? Explain your reasoning. Ron's Work $$\begin{aligned}(f \circ g)(x) &=f(g(x)) \\\&=\left(x^{2}+2\right)^{2}+x-6 \\\&=x^{4}+4 x^{2}+4+x-6 \\\&=x^{4}+4 x^{2}+x-2\end{aligned}$$ Christine's Work\((f \circ g)(x)=f(g(x))\) $$\begin{array}{l}=\left(x^{2}+2\right)^{2}+\left(x^{2}+2\right)-6 \\\=x^{4}+4 x^{2}+4+x^{2}+2-6 \\\=x^{4}+5 x^{2}\end{array}$$

Short answer

Christine is correct. The composite function is \(x^4 + 5x^2\).

Step-by-step solution

Understand the Definitions

Given are two functions, \(f(x) = x^2 + x - 6\) and \(g(x) = x^2 + 2\). A composite function \((f \, \circ \, g)(x)\) means to find \(f(g(x))\), which requires substituting \(g(x)\) into \(f(x)\).

Substitute \(g(x)\) into \(f(x)\)

First, we find \(f(g(x))\) by substituting \(g(x) = x^2 + 2\) into the equation \(f(x) = x^2 + x - 6\). This results in: \[ f(g(x)) = f(x^2 + 2) \rightarrow (x^2 + 2)^2 + (x^2 + 2) - 6 \]

Evaluate \((x^2 + 2)^2\)

Compute \((x^2 + 2)^2\): \[ (x^2 + 2)^2 = x^4 + 4x^2 + 4 \]

Combine Terms

Combine the terms to form \(f(g(x))\): \[ f(g(x)) = x^4 + 4x^2 + 4 + x^2 + 2 - 6 \]

Simplify the Expression

Combine like terms to get the final simplified form: \[ x^4 + 4x^2 + x^2 + 4 + 2 - 6 = x^4 + 5x^2 \]

Compare Both Solutions

Ron's work: \[ x^4 + 4x^2 + x - 2 \] Christine's work: \[ x^4 + 5x^2 \] Based on the steps, Christine is correct because her expression simplifies correctly to the composite function \(x^4 + 5x^2\).

Key concepts

Function Composition

Function composition, written as \(f \circ \ g(x)\), involves plugging one function into another. This means that you start with one function and then apply another function to the result. If we have two functions, f and g, then the composite function \(f \circ \ g(x)\) is \(f(g(x))\). This process helps us combine two functions into a single function, allowing us to analyze how the two functions interact together.
For instance, if \(f(x) = x^2 + x - 6\) and \(g(x) = x^2 + 2\), to find the composite function \(f(g(x))\), you first compute \(g(x)\) and then substitute that result into \(f(x)\). This will allow you to understand how the input of one function affects the output of the combined structure.

Substitution in Functions

Substitution in functions is a crucial step in finding composite functions. It involves taking the expression for one function and replacing its variable with the entire expression of another function. For example, if \(g(x) = x^2 + 2\) and we need to find \(f(g(x))\) where \(f(x) = x^2 + x - 6\), we replace x in \(f(x)\) with the entire expression for \(g(x)\).
This results in: \[ f(g(x)) = \(f(x^2 + 2)\) \rightarrow ((x^2 + 2)^2 + (x^2 + 2) - 6)\] The substitution step is critical because if done incorrectly, it can lead to errors in further simplification and final results. Correct substitution ensures we accurately capture how one function modifies the input the other function provides.

Polynomial Simplification

Polynomial simplification involves combining like terms and reducing expressions to their simplest form. After substitution, the next step is simplifying the resulting polynomial expression.
Let’s take our earlier expression \[ (x^2 + 2)^2 + (x^2 + 2) - 6 \] First, compute \[ (x^2 + 2)^2 = x^4 + 4x^2 + 4 \] Then substitute back in: \[ f(g(x)) = x^4 + 4x^2 + 4 + x^2 + 2 - 6 \] Now, simplify by combining like terms: \[ f(g(x)) = x^4 + 4x^2 + x^2 + 4 + 2 - 6 = x^4 + 5x^2 \] Notice how we first expanded the squared binomial and then collected all similar terms together. Simplifying polynomials is crucial as it helps us get the final, simplest form of the expression.

Algebraic Operations

Algebraic operations, such as addition, subtraction, multiplication, and division, are fundamental tools used during function composition and polynomial simplification. Here, we'll focus on addition and multiplication since they are pivotal in our current context.
When simplifying \[ (x^2 + 2)^2 \], we perform multiplication: \[ (x^2 + 2)(x^2 + 2) = x^4 + 2x^2 + 2x^2 + 4 = x^4 + 4x^2 + 4 \] Similarly, understanding addition is essential when combining terms like: \[ x^4 + 4x^2 + x^2 + 4 + 2 - 6 \] Combine the similar terms to get: \[ x^4 + 5x^2 \] Understanding these operations ensures that every step of simplifying the polynomial is accurate, eventually leading us to the right solution.