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Ch. 8 - Integration Techniques
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 8, Problema 8.4.23

7-56. Trigonometric substitutions Evaluate the following integrals using trigonometric substitution.
23. ∫ 1/(25 - x²)^(3/2) dx

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Step 1: Recognize the integral involves a term of the form \( a^2 - x^2 \), which suggests using the trigonometric substitution \( x = a \sin(\theta) \). Here, \( a = 5 \) because \( 25 = 5^2 \). Substitute \( x = 5 \sin(\theta) \), and compute \( dx = 5 \cos(\theta) d\theta \).
Step 2: Substitute \( x = 5 \sin(\theta) \) and \( dx = 5 \cos(\theta) d\theta \) into the integral. The term \( 25 - x^2 \) becomes \( 25 - 25 \sin^2(\theta) \), which simplifies to \( 25 \cos^2(\theta) \) using the Pythagorean identity \( \sin^2(\theta) + \cos^2(\theta) = 1 \).
Step 3: Rewrite the integral in terms of \( \theta \): \( \int \frac{1}{(25 \cos^2(\theta))^{3/2}} \cdot 5 \cos(\theta) d\theta \). Simplify the denominator \( (25 \cos^2(\theta))^{3/2} \) to \( 125 \cos^3(\theta) \). The integral becomes \( \int \frac{5 \cos(\theta)}{125 \cos^3(\theta)} d\theta \).
Step 4: Simplify the fraction \( \frac{5 \cos(\theta)}{125 \cos^3(\theta)} \) to \( \frac{1}{25 \cos^2(\theta)} \). The integral now becomes \( \int \frac{1}{25 \cos^2(\theta)} d\theta \), which can be rewritten as \( \frac{1}{25} \int \sec^2(\theta) d\theta \).
Step 5: Evaluate \( \int \sec^2(\theta) d\theta \), which is a standard integral equal to \( \tan(\theta) \). After finding \( \tan(\theta) \), use the substitution \( \theta = \arcsin(x/5) \) to express the result back in terms of \( x \).

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Trigonometric Substitution

Trigonometric substitution is a technique used in calculus to simplify integrals involving square roots of quadratic expressions. By substituting a variable with a trigonometric function, such as x = a sin(θ) or x = a tan(θ), the integral can often be transformed into a more manageable form. This method is particularly useful for integrals that contain expressions like √(a² - x²), √(x² + a²), or √(x² - a²).
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Pythagorean Identity

The Pythagorean identity, sin²(θ) + cos²(θ) = 1, is a fundamental relationship in trigonometry that is often used in conjunction with trigonometric substitution. This identity helps simplify expressions involving trigonometric functions, especially when converting back to the original variable after integration. Recognizing how to manipulate this identity is crucial for solving integrals that arise from trigonometric substitutions.
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