Problem 62 - A Graphical Approach to College Algebra

Question

Use \(f(x)\) and \(g(x)\) to find each composition. Identify its domain. (Use a calculator if necessary to find the domain.) (a) \((f \circ g)(x) \quad\) (b) \((g \circ f)(x) \quad\) (c) \((f \circ f)(x)\). $$f(x)=x+2, \quad g(x)=x^{4}+x^{2}-3 x-4$$

Step-by-Step Solution

Answer

(a) \((f \circ g)(x) = x^4 + x^2 - 3x - 2\), (b) \((g \circ f)(x) = x^4 + 8x^3 + 25x^2 + 33x + 10\), (c) \((f \circ f)(x) = x + 4\). Domain is all real numbers.

Step 1: Find \((f \circ g)(x)\)

The composition \((f \circ g)(x)\) means substitute \(g(x)\) into \(f(x)\). Given \(f(x) = x + 2\), substitute \(g(x) = x^4 + x^2 - 3x - 4\) into \(f(x)\): \[f(g(x)) = g(x) + 2 = x^4 + x^2 - 3x - 4 + 2 = x^4 + x^2 - 3x - 2\] Therefore, \((f \circ g)(x) = x^4 + x^2 - 3x - 2\). Since \(g(x)\) is a polynomial, its domain is all real numbers.

Step 2: Find \((g \circ f)(x)\)

The composition \((g \circ f)(x)\) means substitute \(f(x)\) into \(g(x)\). Lift \(f(x) = x + 2\), replace in \(g(x) = x^4 + x^2 - 3x - 4\): \[g(f(x)) = (x+2)^4 + (x+2)^2 - 3(x+2) - 4\]Now expand and simplify \[(x+2)^4 = x^4 + 8x^3 + 24x^2 + 32x + 16\]\[(x+2)^2 = x^2 + 4x + 4\]Substitute back:\[x^4 + 8x^3 + 24x^2 + 32x + 16 + x^2 + 4x + 4 - 3x - 6 - 4\]Combine like terms: \[g(f(x)) = x^4 + 8x^3 + 25x^2 + 33x + 10\]Therefore, \((g \circ f)(x) = x^4 + 8x^3 + 25x^2 + 33x + 10\). The domain is all real numbers since it remains a polynomial.

Step 3: Find \((f \circ f)(x)\)

The composition \((f \circ f)(x)\) requires substituting \(f(x)\) back into itself, \(f(x) = x + 2\): \[f(f(x)) = f(x + 2) = (x + 2) + 2 = x + 4\]Thus, \((f \circ f)(x) = x + 4\). As it is linear, like \(f(x)\), the domain is also all real numbers.

Key Concepts

Domain of Functions

The domain of a function is the set of all possible input values (usually denoted as \(x\)) that will give a valid output for the function. Understanding the domain of functions is crucial because it tells us where the function is defined.

For polynomial functions, which include expressions like \(x^4 + x^2 - 3x - 4\), the domain is typically all real numbers. This is because polynomial functions do not have restrictions such as division by zero or square roots of negative numbers, both of which can limit the domain in other types of functions. Therefore, when dealing with compositions like \((f \circ g)(x)\), \((g \circ f)(x)\), and \((f \circ f)(x)\), where \(f(x)\) and \(g(x)\) are polynomial functions, their domains remain all real numbers.

• For \( (f \circ g)(x) = x^4 + x^2 - 3x - 2 \), the domain is all real numbers.
• Similarly, \((g \circ f)(x) = x^4 + 8x^3 + 25x^2 + 33x + 10\) is also defined for all real numbers.
• For the composition \((f \circ f)(x) = x + 4\), because it is a linear polynomial, its domain is again all real numbers.

Recognizing the domain helps us to apply and manipulate functions without inadvertently causing errors in calculations.

Polynomial Functions

Polynomial functions are an essential class of functions that are represented by expressions involving variables raised to whole number powers and constants. The general form of a polynomial function is \(a_nx^n + a_{n-1}x^{n-1} + \ldots + a_1x + a_0\), where the coefficients \(a_n, a_{n-1}, \ldots, a_0\) are constants, and the exponents are whole numbers.

Let's break down a few important points about polynomial functions:

• Polynomials can have varying degrees, which is determined by the highest power of the variable. For instance, \(g(x) = x^4 + x^2 - 3x - 4\) is a degree 4 polynomial.
• They are continuous and differentiable over their entire domain (all real numbers).
• Polynomials can be combined through addition, subtraction, multiplication, and composition, often resulting in another polynomial.

When you perform operations like substitution within polynomials, as when calculating \((g \circ f)(x)\), each term of the polynomial gets expanded according to rules of exponentiation and distribution, ensuring the resulting expression is again a polynomial.

Function Operations

Function operations involve combining or manipulating functions in different ways. The main types of operations on functions include addition, subtraction, multiplication, division, and composition. Composition, in particular, is a method where the output of one function becomes the input for another.

To perform composition, we use the notation \((f \circ g)(x)\), which means you substitute \(g(x)\) into \(f(x)\). It's crucial to follow each calculation step carefully to avoid errors.

• When dealing with \((f \circ g)(x)\), it results in substituting the entire expression for \(g(x)\) into \(f(x)\). The calculation results in a new function.
• Similarly, \((g \circ f)(x)\) involves inserting \(f(x)\) into \(g(x)\)'s function equation, requiring expansion and simplification.
• For \((f \circ f)(x)\), the process involves substituting \(f(x)\) into itself, which is straightforward when \(f(x)\) is a linear polynomial.

Practical applications of function operations, including compositions, extend to modeling and solving real-world problems, where one process depends on the outcome of another. It's essential to maintain accuracy in calculations to derive the correct model or solution.