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# Integration

## Introduction

Integration is the second primary operation in calculus, with differentiation being the first. While differentiation measures rates of change, integration accumulates quantities. Integrals can be used to determine areas under curves, volumes of solids of revolution, solutions to differential equations, and more.

## Basic Definition

Given a function $$f(x)$$, the integral of $$f$$ is represented by the symbol $$\int f(x) \, dx$$. The process of finding the integral is called integration.

There are two main types of integrals:

1. Indefinite Integrals: These are integrals that represent families of functions whose derivatives are the function being integrated. The result of an indefinite integral is always accompanied by a constant of integration, usually denoted as $$C$$.
2. Definite Integrals: These integrals compute the net area between the function and the x-axis over a specified interval $$[a, b]$$. The result of a definite integral is a number.

## Basic Integration Rules

### 1. Integration of a Constant:

$$\int c \, dx = cx + C$$ (where $$c$$ is a constant)

### 2. Addition (and Subtraction) Rule for Integration:

The addition rule for integration states that the integral of a sum is the sum of the integrals. Similarly, the integral of a difference is the difference of the integrals.

For functions $$f(x)$$ and $$g(x)$$:

$$\int [f(x) + g(x)] \, dx = \int f(x) \, dx + \int g(x) \, dx$$

Subtraction:

$$\int [f(x) – g(x)] \, dx = \int f(x) \, dx – \int g(x) \, dx$$

### Example:

1. Let’s integrate the function $$h(x) = x^3 + x^2$$:
$$\int h(x) \, dx = \int (x^3 + x^2) \, dx$$

Using the addition rule, we separate the terms:

$$\int (x^3 + x^2) \, dx = \int x^3 \, dx + \int x^2 \, dx$$

Upon integrating individually, we get:

$$\int (x^3 + x^2) \, dx = \frac{1}{4}x^4 + \frac{1}{3}x^3 + C$$

Where $$C$$ is the constant of integration.

This demonstrates the application of the addition rule for integration using powers of $$x$$.

### 3. Power Rule for Integration:

For any real number $$n$$ not equal to -1,

$$\int x^n \, dx = \frac{x^{n+1}}{n+1} + C$$

### 4. Integration of Exponential Functions:

$$\int e^x \, dx = e^x + C$$

### 5. Integration of Basic Trigonometric Functions:

$$\int \sin(x) \, dx = -\cos(x) + C$$ $$\int \cos(x) \, dx = \sin(x) + C$$

### 6. Linearity of Integration:

Integration is a linear operation, which means:

$$\int [af(x) + bg(x)] \, dx = a\int f(x) \, dx + b\int g(x) \, dx$$

Where $$a$$ and $$b$$ are constants.

### 1. Integration of a Constant:

Rule: $$\int c \, dx = cx + C$$

Example:

$$\int 5 \, dx$$

Using the rule, we get:

$$\int 5 \, dx = 5x + C$$

### 2. Power Rule for Integration:

Rule: For any real number $$n$$ not equal to -1,

$$\int x^n \, dx = \frac{x^{n+1}}{n+1} + C$$

Example:

$$\int x^4 \, dx$$

Using the rule, we get:

$$\int x^4 \, dx = \frac{x^5}{5} + C$$

### 3. Integration of Exponential Functions:

Rule: $$\int e^x \, dx = e^x + C$$

Example:

$$\int e^{2x} \, dx$$

Using a substitution method (let $$u = 2x$$, so $$du = 2 \, dx$$), we get:

$$\int e^{2x} \, dx = \frac{1}{2}e^{2x} + C$$

### 4. Integration of Basic Trigonometric Functions:

Rule for sine:

$$\int \sin(x) \, dx = -\cos(x) + C$$

Example:

$$\int \sin(3x) \, dx$$

Using substitution (let $$u = 3x$$, so $$du = 3 \, dx$$), we get:

$$\int \sin(3x) \, dx = -\frac{1}{3}\cos(3x) + C$$

Rule for cosine:

$$\int \cos(x) \, dx = \sin(x) + C$$

Example:

$$\int \cos(2x) \, dx$$

Using substitution, we get:

$$\int \cos(2x) \, dx = \frac{1}{2}\sin(2x) + C$$

### 5. Linearity of Integration:

Rule: $$\int [af(x) + bg(x)] \, dx = a\int f(x) \, dx + b\int g(x) \, dx$$

Example:

$$\int [3x^2 + 2\sin(x)] \, dx$$

Using linearity and the integration rules, we get:
$$\int [3x^2 + 2\sin(x)] \, dx = 3\int x^2 \, dx + 2\int \sin(x) \, dx$$

$$= x^3 – 2\cos(x) + C$$

## Fundamental Theorem of Calculus

This theorem connects differentiation and integration.

If $$f(x)$$ is a continuous function on the interval $$[a, b]$$ and $$F(x)$$ is an antiderivative of $$f(x)$$ on $$[a, b]$$, then:

$$\int_a^b f(x) \, dx = F(b) – F(a)$$

## Methods of Integration

There are multiple techniques to evaluate more complex integrals:

1. Substitution Method: It involves making a substitution to simplify an integral. For instance, for the integral $$\int 2x \sin(x^2 + 1) \, dx$$, we can set $$u = x^2 + 1$$, which simplifies the integral.
2. Integration by Parts: This method is derived from the product rule of differentiation. It’s expressed as:
$$\int u \, dv = uv – \int v \, du$$

Partial Fractions: Used to integrate rational functions. It involves expressing the integrand as a sum of simpler fractions and then integrating each of those separately.

Trigonometric Substitution: Useful for integrals involving the square roots of quadratic polynomials.

## Applications of Integration

1. Area Under a Curve: The definite integral can compute the net area between a function and the x-axis over an interval.
2. Volumes of Solids of Revolution: By revolving a region around an axis, we can use integration to determine the volume of the solid formed.
3. Solving Differential Equations: Integrals are often used to solve ordinary differential equations, which describe various phenomena in nature and engineering.

## Conclusion

Integration is a powerful mathematical tool with vast applications across different domains. Together with differentiation, it forms the cornerstone of calculus, paving the way for more advanced mathematical analysis and its applications in various fields.

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