Integral Of Sqrt 9-x 2

Decoding the Integral of √(9 - x²): A Comprehensive Guide

The integral ∫√(9 - x²) dx is a classic example of a definite integral that often appears in calculus courses and various applications, from calculating areas under curves to solving physics problems. This seemingly simple integral requires a strategic approach and understanding of trigonometric substitution to solve effectively. This article will provide a comprehensive walkthrough of the solution, exploring the underlying principles, step-by-step calculations, and related concepts. We'll delve into the details, ensuring a thorough understanding for all readers, regardless of their mathematical background.

Introduction: Understanding the Problem

The integral ∫√(9 - x²) dx presents a challenge because it's not directly solvable using basic integration rules. The presence of the square root term complicates the process. We're tasked with finding a function whose derivative is √(9 - x²). This necessitates a clever technique: trigonometric substitution. This method leverages the trigonometric identities to transform the integral into a more manageable form. The specific choice of substitution is crucial and directly relates to the form of the expression under the square root. In this case, the form 9 - x² hints at a trigonometric identity involving a difference of squares.

Step-by-Step Solution Using Trigonometric Substitution

The key to solving this integral lies in recognizing the similarity to the Pythagorean identity: sin²(θ) + cos²(θ) = 1. We can manipulate this identity to create a substitution that simplifies the integral.

1. Choosing the Substitution: Observe that 9 - x² resembles 3² - x². This suggests the substitution x = 3sin(θ). This choice is motivated by the fact that 9 - (3sin(θ))² simplifies neatly using the Pythagorean identity.

2. Finding dx: Differentiate the substitution x = 3sin(θ) with respect to θ to find dx: dx = 3cos(θ) dθ

3. Substituting into the Integral: Now, replace x and dx in the original integral with their equivalents in terms of θ: ∫√(9 - x²) dx = ∫√(9 - (3sin(θ))²) * 3cos(θ) dθ

4. Simplifying the Integrand: Using the Pythagorean identity (1 - sin²(θ) = cos²(θ)), we simplify the expression under the square root: √(9 - (3sin(θ))²) = √(9(1 - sin²(θ))) = √(9cos²(θ)) = 3cos(θ)

   Therefore, the integral becomes: ∫(3cos(θ))(3cos(θ)) dθ = ∫9cos²(θ) dθ

5. Using a Trigonometric Identity: The integral now contains cos²(θ), which isn't directly integrable. We employ the double-angle identity: cos²(θ) = (1 + cos(2θ))/2. Substituting this into the integral gives:

   ∫9cos²(θ) dθ = ∫9(1 + cos(2θ))/2 dθ = (9/2)∫(1 + cos(2θ)) dθ

6. Integrating: Now we can integrate term by term: (9/2)∫(1 + cos(2θ)) dθ = (9/2)[θ + (1/2)sin(2θ)] + C (where C is the constant of integration)

7. Using the Double-Angle Identity (Again): To express the result in terms of x, we need to rewrite sin(2θ) using the double-angle identity: sin(2θ) = 2sin(θ)cos(θ). This yields:

   (9/2)[θ + sin(θ)cos(θ)] + C

8. Returning to x: Remember our initial substitution x = 3sin(θ). This implies sin(θ) = x/3. To find cos(θ), we can use the Pythagorean identity: cos²(θ) = 1 - sin²(θ) = 1 - (x/3)² = (9 - x²)/9. Therefore, cos(θ) = √(9 - x²)/3. Also, θ = arcsin(x/3).

9. Final Result: Substituting these back into the expression, we get the final result:

   (9/2)[arcsin(x/3) + (x/3)(√(9 - x²)/3)] + C = (9/2)arcsin(x/3) + (x/2)√(9 - x²) + C

Explanation of the Trigonometric Substitution

The core idea behind trigonometric substitution is to transform the integrand into a form that can be easily integrated using standard trigonometric identities. The choice of substitution depends on the form of the expression inside the square root. In this case:

• √(a² - x²): The substitution x = a sin(θ) is used. This is because 1 - sin²(θ) = cos²(θ). The constant 'a' is the square root of the constant term, in this case, 3.

• √(a² + x²): The substitution x = a tan(θ) is used. This leverages the identity 1 + tan²(θ) = sec²(θ).

• √(x² - a²): The substitution x = a sec(θ) is used. This uses the identity sec²(θ) - 1 = tan²(θ).

The process involves:

1. Choosing the appropriate substitution based on the form of the expression under the square root.
2. Differentiating the substitution to find dx in terms of dθ.
3. Substituting x and dx into the integral.
4. Simplifying the integrand using trigonometric identities.
5. Integrating the simplified expression.
6. Substituting back to express the result in terms of the original variable, x.

Geometric Interpretation

The integral ∫√(9 - x²) dx represents the area of a semicircle with radius 3. This is because the equation x² + y² = 9 describes a circle with radius 3, and the function y = √(9 - x²) represents the upper half of that circle. The integral calculates the area of this semicircle between the limits of integration. If the integral is definite, with limits from -3 to 3, the result will be (1/2)π(3)² = (9π/2), which is consistent with the area of a semicircle with radius 3.

Frequently Asked Questions (FAQ)

Q1: What if the constant under the square root is not a perfect square?

A1: If the constant is not a perfect square, you'll still use a similar trigonometric substitution. For example, if the integral was ∫√(5 - x²) dx, you would use x = √5 sin(θ). The process remains the same, just with slightly more complex arithmetic.

Q2: Are there alternative methods to solve this integral?

A2: While trigonometric substitution is the most efficient method for this particular integral, other techniques might be applicable in some cases. However, these methods are often more complex and less direct.

Q3: What happens if the limits of integration are not from -3 to 3?

A3: If the integral has definite limits other than -3 to 3, you substitute the limits of integration into the final expression in terms of x and evaluate the difference. The result will represent the area under the curve √(9 - x²) within those specified limits.

Q4: Why is the constant of integration (C) added?

A4: The constant of integration (C) is added because the derivative of a constant is zero. Therefore, there are infinitely many functions whose derivative is √(9 - x²), differing only by a constant.

Conclusion: Mastering Trigonometric Substitution

The integral ∫√(9 - x²) dx serves as an excellent example of the power and elegance of trigonometric substitution in calculus. This technique is a valuable tool for solving a wide range of integrals involving expressions under square roots. Mastering this technique will significantly enhance your ability to tackle more complex integration problems. The key takeaway is recognizing the appropriate trigonometric identity to use based on the form of the expression inside the square root, and then meticulously following the steps of substitution, simplification, integration, and back-substitution to obtain the final result. Remember to always check your answer and consider the geometric interpretation to gain a deeper understanding of the problem. This detailed explanation should help you not only solve this specific integral but also approach similar problems with confidence and understanding.