Problem 75 - Calculus for Biology and Medicine

Question

Compute the indefinite integrals. $$ \int \frac{4}{1+x^{2}} d x $$

Step-by-Step Solution

Answer

The integral is \( 4 \tan^{-1}(x) + C \).

Step 1: Recognize the Integral Form

The integral \( \int \frac{4}{1+x^{2}} \, dx \) resembles the standard form \( \int \frac{1}{1+x^{2}} \, dx \), which is known to be the integral that results in the inverse tangent function, also called the arctangent.

Step 2: Factor Out the Constant

Factor out the constant 4 from the integral using the property of integrals that allows constants to be factored out. This gives us: \( 4 \int \frac{1}{1+x^{2}} \, dx \).

Step 3: Integrate Using the Arctangent Formula

Apply the standard integration formula \( \int \frac{1}{1+x^{2}} \, dx = \tan^{-1}(x) + C \), where \( C \) is the constant of integration. Thus, integrate to obtain \( 4 \tan^{-1}(x) + C \).

Key Concepts

Inverse Tangent Function

The inverse tangent function, often denoted as \( \tan^{-1}(x) \) or \( \arctan(x) \), is a fundamental concept in calculus, particularly in integration. This function essentially reverses what the tangent function does. The tangent function takes an angle and returns a ratio, while the inverse tangent function takes that ratio and returns the angle.
The function \( \tan^{-1}(x) \) involves finding the angle whose tangent is \( x \). It has a range of \((-\pi/2, \pi/2)\), making it continuous and invertible within this interval. Understanding this function is crucial when dealing with integrals that resemble the form \( \int \frac{1}{1+x^{2}} \), where the solution is linked to the inverse tangent function.

Arctangent

Arctangent is simply another name for the inverse tangent function and is written as \( \text{arctan}(x) \). This term is often used interchangeably with \( \tan^{-1}(x) \) in calculus problems. Its significance comes from its role as the antiderivative in specific integrals.
Whenever we encounter integrals like \( \int \frac{1}{1+x^{2}} \, dx \), they are essentially calling for the application of arctangent. Recognizing this helps solve the integral efficiently.

• The result of the integral \( \int \frac{1}{1+x^{2}} \, dx \) is \( \text{arctan}(x) + C \).
• Constants like 4 can be factored out of the integral, maintaining focus on the arctangent operation.

This simplicity of integration using arctangent is one of the many reasons it is favored in calculus.

Integration Formula

In integration, certain formulas are pivotal for solving indefinite integrals efficiently. One such formula is for the inverse tangent function. For the integral \( \int \frac{1}{1+x^{2}} \, dx \), the result is straightforward: \( \tan^{-1}(x) + C \).
This formula is fundamental because it provides a direct way to integrate expressions in this specific form. It's worth noting the following when using this formula:

• Recognize the structure of the integral to decide on directly applying the formula.
• The constant \( C \) represents the constant of integration, necessary for indefinite integrals.
• Such formulas act as shortcuts, streamlining problem-solving processes.

By understanding and applying this integration formula, tasks involving specific forms like \( \int \frac{4}{1+x^{2}} \, dx \) become more straightforward, making it easier to focus on the essential concepts of calculus.