Ch. 5 - Integration

Briggs - Calculus: Early Transcendentals 3rd Edition

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Briggs 3rd Edition

Ch. 5 - Integration

Problem 5.4.49

Chapter 5, Problem 5.4.49

Symmetry of composite functions Prove that the integrand is either even or odd. Then give the value of the integral or show how it can be simplified. Assume f and g are even functions and p and q are odd functions.

∫ᵃ₋ₐ ƒ(g(𝓍)) d𝓍

Verified step by step guidance

Step 1: Recall the definitions of even and odd functions. An even function satisfies f(x) = f(-x), while an odd function satisfies p(x) = -p(-x). These properties will be crucial in analyzing the symmetry of the composite function f(g(x)).

Step 2: Analyze the composition f(g(x)). Since f(x) is even, f(g(x)) will inherit the evenness of g(x) if g(x) is also even. Specifically, g(x) = g(-x) implies f(g(x)) = f(g(-x)), making f(g(x)) an even function.

Step 3: Consider the integral ∫ᵃ₋ₐ f(g(x)) dx. If f(g(x)) is even, the integral over the symmetric interval [-a, a] simplifies to 2 * ∫₀ᵃ f(g(x)) dx because the contributions from [-a, 0] and [0, a] are identical.

Step 4: If g(x) were odd, g(-x) = -g(x). Substituting this into f(g(x)), we would need to check whether f(-g(x)) = -f(g(x)) (odd) or f(-g(x)) = f(g(x)) (even). This determines whether f(g(x)) is odd or even.

Step 5: Conclude the symmetry analysis. If f(g(x)) is odd, the integral ∫ᵃ₋ₐ f(g(x)) dx evaluates to 0 because the contributions from [-a, 0] and [0, a] cancel each other out. If f(g(x)) is even, simplify the integral as described in Step 3.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Even and Odd Functions

Even functions are symmetric about the y-axis, meaning f(-x) = f(x) for all x. Odd functions have rotational symmetry about the origin, satisfying f(-x) = -f(x). Understanding these properties is crucial for analyzing the symmetry of composite functions and determining the nature of the integrand.

Composite Functions

A composite function is formed when one function is applied to the result of another function, denoted as (f ∘ g)(x) = f(g(x)). In the context of the integral, knowing how to evaluate the symmetry of composite functions helps in determining whether the integrand is even or odd, which influences the integral's value.

Properties of Integrals

The integral of an even function over a symmetric interval [-a, a] is twice the integral from 0 to a, while the integral of an odd function over the same interval is zero. These properties simplify the evaluation of integrals and are essential for proving the nature of the integrand in the given problem.

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