Problem 27 - Calculus Early Transcendentals: Pearson New International Edition

Question

Perform the indicated integrations. $$ \int \tan ^{3} x \sec ^{2} x d x $$

Step-by-Step Solution

Answer

The integral evaluates to \( \frac{\tan^4 x}{4} + C \).

Step 1: Recognize the Integral Form

The integral given is \( \int \tan^{3} x \sec^{2} x \, dx \). This can be expressed as \( \int \tan^{n} x \sec^{2} x \, dx \) where \( n = 3 \). This form is suitable for substitution since the derivative of \( \tan x \) is \( \sec^2 x \).

Step 2: Identify the Substitution

Let \( u = \tan x \). Then, the derivative of \( u \) with respect to \( x \) is \( du = \sec^2 x \, dx \). This allows us to substitute \( u \) and \( du \) into the integral.

Step 3: Substitute and Simplify

Substitute into the integral to get the form \( \int u^3 \, du \). This transforms the problem into a standard power integral.

Step 4: Perform the Integration

Integrate using the power rule \( \int u^n \, du = \frac{u^{n+1}}{n+1} + C \). For \( u^3 \), this becomes \( \frac{u^4}{4} + C \).

Step 5: Substitute Back

Replace \( u \) with \( \tan x \) to return to the variable \( x \). Thus, \( \frac{u^4}{4} + C \) becomes \( \frac{\tan^4 x}{4} + C \).

Key Concepts

Substitution Method

The substitution method is a cornerstone of integration techniques in calculus, designed to simplify the integration process, especially when dealing with complex functions. It's a method akin to reverse chain rule application. Suppose you have an integral where a function's derivative is also present. In that case, substitution can effectively transform the integral into a simpler form.

In our example, the integral \( \int \tan^{3} x \sec^{2} x \, dx \), substitution is employed because the presence of \( \sec^{2} x \, dx \) is the derivative of \( \tan x \).

• You start by choosing a substitution that simplifies the integral, making it easier to work with standard integration formulas.
• Letting \( u = \tan x \) results in \( du = \sec^{2} x \, dx \). This substitution turns the integration into a basic power integral: \( \int u^3 \, du \).
• This transformation is advantageous because integrating \( u^3 \) is straightforward, with the power rule being easily applicable.

Once the integral is computed, it's crucial to substitute back the original variable, ensuring the solution matches the original integral's variable.

Trigonometric Integrals

Trigonometric integrals are integrals that involve trigonometric functions such as sine, cosine, tangent, and secant. These integrals often require specialized techniques like substitution to solve efficiently.

In the provided problem, \( \int \tan^{3} x \sec^{2} x \, dx \), the functions \( \tan x \) and \( \sec x \) are involved. Here’s what you typically do with trigonometric integrals:

• Identify familiar derivative relationships, like \( \frac{d}{dx}(\tan x) = \sec^2 x \), which are invaluable for substitution.
• Exploit identities and derivatives of trigonometric functions to transform the integral into a more manageable form.
• Convert challenging products of trigonometric functions into simpler, integrable powers by using identities as needed.

These strategies harness known formulas and identities, simplifying the otherwise complex process of handling integrals involving trigonometric products.

Calculus Problem Solving

Solving calculus problems often involves a structured approach, using techniques and strategies tailored to the problem type. Here are the steps followed in the trigonometric integral problem such as \( \int \tan^{3} x \sec^{2} x \, dx \):

• **Recognize Patterns**: Identifying patterns such as trigonometric derivatives (\( \sec^2 x \) being the derivative of \( \tan x \)) is critical for selecting the right integration technique.
• **Choose the Right Technique**: Selecting substitution when you see a derivative of one of the terms present simplifies the problem significantly.
• **Organize the Steps**: Clearly define each step such as substitution, integration, and re-substitution to ensure correctness.
• **Double-Check Your Work**: Once you've found the antiderivative, it’s crucial to substitute back and check against any bounds or conditions given in the problem.

By following these logical, structured steps, you can effectively tackle a broad range of integration problems in calculus. Practice and familiarity with various techniques, such as substitution, are essential to humanizing these seemingly daunting challenges into manageable tasks.