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Ch. 8 - Techniques of Integration
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Not the one you use?Change textbook
All textbooksHass 15th EditionCh. 8 - Techniques of IntegrationProblem 8.8.92
Chapter 8, Problem 8.8.92

92. Evaluate ∫ from 3 to ∞ [ dx / (x √(x² - 9))]

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Recognize that the integral is an improper integral because the upper limit is infinity. So, rewrite the integral as a limit: \(\displaystyle \lim_{t \to \infty} \int_{3}^{t} \frac{dx}{x \sqrt{x^{2} - 9}}\).
To evaluate the integral \(\int \frac{dx}{x \sqrt{x^{2} - 9}}\), use a trigonometric substitution. Since the integrand contains \(\sqrt{x^{2} - 9}\), let \(x = 3 \sec \theta\), which implies \(dx = 3 \sec \theta \tan \theta \, d\theta\).
Substitute \(x = 3 \sec \theta\) and \(dx\) into the integral. Also, express \(\sqrt{x^{2} - 9}\) in terms of \(\theta\): \(\sqrt{(3 \sec \theta)^{2} - 9} = \sqrt{9 \sec^{2} \theta - 9} = 3 \tan \theta\).
Rewrite the integral in terms of \(\theta\): \(\int \frac{3 \sec \theta \tan \theta \, d\theta}{3 \sec \theta \cdot 3 \tan \theta}\). Simplify the expression by canceling common factors.
After simplification, integrate the resulting expression with respect to \(\theta\). Then, convert back to the variable \(x\) using the inverse trigonometric relationships from the substitution. Finally, apply the limits by converting the original limits \(x=3\) and \(x=t\) to their corresponding \(\theta\) values, and take the limit as \(t \to \infty\).

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Key Concepts

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

Improper Integrals

Improper integrals involve integration over an infinite interval or integrands with infinite discontinuities. To evaluate them, we replace the infinite limit with a variable and take the limit as it approaches infinity, ensuring the integral converges to a finite value.
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Improper Integrals: Infinite Intervals

Trigonometric Substitution

Trigonometric substitution is a technique used to simplify integrals involving expressions like √(x² - a²). By substituting x = a sec(θ), the radical simplifies using trigonometric identities, making the integral easier to evaluate.
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Introduction to Trigonometric Functions

Definite Integral Evaluation

Evaluating a definite integral requires finding the antiderivative and then applying the Fundamental Theorem of Calculus by substituting the upper and lower limits. For improper integrals, this includes taking limits at infinite bounds to determine convergence.
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Definition of the Definite Integral
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