# Integration by parts

##### Intros
###### Lessons
1. How to do integration by parts.
##### Examples
###### Lessons
1. Evaluate: $\smallint x\cos x{d}x$
###### Topic Notes
In this section, we will learn how to integrate a product of two functions using integration by parts. This technique requires you to choose which function is substituted as "u", and which function is substituted as "dv". We will also take a look at two special cases. The first case is integrating a function which seems to not be a product of two functions. However, we see that we can actually have a product of two functions if we set "dv" as "dx". The second special case involves using integration by parts several times to get the answer.

## Introduction to Integration by Parts

Integration by parts is a powerful technique in calculus that allows us to integrate the product of two functions. This method is derived from the product rule in differential calculus, making it a crucial tool for solving complex integrals. Our introduction video provides a comprehensive overview of this technique, serving as an essential starting point for students learning advanced integration methods. By understanding integration by parts, you'll be able to tackle a wide range of integrals that would otherwise be challenging or impossible to solve using basic integration techniques. This method is particularly useful when dealing with products of algebraic and transcendental functions, logarithmic functions, or trigonometric functions multiplied by polynomials. As you progress in your calculus studies, you'll find that integration by parts becomes an indispensable tool in your mathematical toolkit, enabling you to solve increasingly complex problems and gain a deeper understanding of integral calculus.

## When to Use Integration by Parts

Integration by parts is a powerful technique in calculus that proves most useful when dealing with integrals involving the product of two functions, particularly when one function is a polynomial and transcendental functions. This method shines when integrating combinations of polynomial and transcendental functions with trigonometric, exponential, or logarithmic functions.

The technique is especially effective when one part of the integrand becomes simpler after differentiation, while the other part remains manageable after integration. This characteristic makes integration by parts ideal for certain types of integrals that would otherwise be challenging to solve using other methods.

Let's consider some examples where integration by parts is the optimal approach:

1. Integrals involving polynomials and exponential functions, such as x²e^x dx or xe^(-x) dx. In these cases, the polynomial part simplifies through differentiation, while the exponential function remains relatively unchanged after integration.

2. Integrals combining polynomials and trigonometric functions, like x sin(x) dx or x² cos(x) dx. Here, the polynomial simplifies through differentiation, and the trigonometric function cycles through its derivatives, eventually leading to a solvable form.

3. Integrals with logarithmic functions multiplied by polynomials, such as x ln(x) dx or x² ln(x) dx. The logarithmic function simplifies when differentiated, while the polynomial part integrates easily.

4. Integrals involving inverse trigonometric functions and polynomials, like x arctan(x) dx or x² arcsin(x) dx. These benefit from integration by parts as the inverse trigonometric function simplifies when differentiated.

Integration by parts is particularly useful when dealing with products where one function doesn't have an elementary antiderivative on its own, but becomes integrable when multiplied by another function and integrated by parts. This technique often transforms complex integrals into simpler ones, sometimes requiring multiple applications to reach a final solution.

However, it's important to note that integration by parts is not always the best choice for every integral. For instance, it's generally not suitable for integrals involving radical functions, such as x dx or (1/x) dx. These types of integrals are usually better handled by substitution methods or other specialized techniques.

Additionally, integration by parts may not be efficient for integrals where both functions become more complicated after applying the technique, or when the resulting integral is similar to or more complex than the original. In such cases, alternative methods like trigonometric substitution, partial fractions, or advanced techniques may be more appropriate.

To effectively use integration by parts, it's crucial to recognize the types of integrands that benefit from this method. Practice and experience help in developing the intuition to identify when integration by parts is the most efficient approach. Remember, the goal is to simplify the integral, so choosing which function to differentiate and which to integrate is key to successfully applying this technique.

In conclusion, integration by parts is a versatile tool in the calculus toolkit, particularly adept at handling integrals involving products of polynomial and transcendental functions. Its effectiveness in simplifying complex integrals makes it an essential technique for students and practitioners of mathematics alike. By understanding when and how to apply integration by parts, one can tackle a wide range of challenging integrals with confidence and precision.

## The Formula for Integration by Parts

Integration by parts is a fundamental technique in calculus that allows us to solve complex integrals by breaking them down into simpler components. The formula for integration by parts comes in two versions: a longer, more detailed form and a shorter, more concise form. Both are essential for students to understand and apply in various mathematical problems.

The longer version of the integration by parts formula is:

f(x)g'(x)dx = f(x)g(x) - f'(x)g(x)dx

This formula provides a clear breakdown of the integration process, showing how the original integral is transformed into a new expression and a new integral. However, mathematicians and students often prefer to work with a more compact version of this formula.

The shorter, more commonly used version of the integration by parts formula is:

u dv = uv - v du

This concise form is derived from the longer version through a simple substitution process. We make the following substitutions:

• u = f(x)
• dv = g'(x)dx

From these substitutions, we can deduce that:

• du = f'(x)dx
• v = g(x)

By applying these substitutions to the longer formula, we arrive at the shorter version. This compact form is not only easier to remember but also more practical to apply in various integration problems.

It's crucial for students to memorize the shorter version of the integration by parts formula, as it may not be provided on exams or in certain mathematical contexts. To aid in memorization, a helpful mnemonic device is:

"u dv = uv - v du"

This mnemonic encapsulates the essence of the formula, with the integral signs at the beginning and end serving as visual cues. The symmetry of the expression (u dv on one side, v du on the other) makes it easier to recall in high-pressure situations like exams.

When applying the integration by parts technique, the choice of u and dv is critical. Generally, we choose u to be the function that becomes simpler when differentiated, and dv to be the function that remains integrable when integrated. This strategic selection often leads to a simpler integral on the right-hand side of the equation, making the overall problem more manageable.

The integration by parts formula is particularly useful for integrals involving products of functions, especially when one function becomes simpler upon differentiation (like logarithmic or inverse trigonometric functions) and the other becomes more complex upon integration (like polynomials or exponential functions).

In practice, integration by parts often requires multiple applications to solve a single problem. Each application can potentially simplify the integral further, eventually leading to a solvable form. This iterative process highlights the power and versatility of the technique in handling complex integrals that would be difficult or impossible to solve through other methods.

Understanding and mastering the integration by parts formula is essential for success in calculus and higher-level mathematics. It's a tool that finds applications not only in pure mathematics but also in various fields of science and engineering, where complex integrals frequently arise in modeling real-world phenomena.

In conclusion, the integration by parts formula, particularly in its shorter form u dv = uv - v du, is a cornerstone of integral calculus. Its derivation from the longer form through substitution demonstrates the elegance of mathematical reasoning. By memorizing and understanding this formula, students equip themselves with a powerful tool for solving a wide range of integrals, paving the way for deeper exploration of mathematics and its applications.

## Applying Integration by Parts: Step-by-Step Guide

Integration by parts is a powerful technique used to solve complex integrals. This step-by-step guide will walk you through the process of applying the integration by parts formula, including tips on choosing 'u' and 'dv', finding 'v', and setting up the resulting integral. Let's dive in!

Step 1: Understand the Formula
The integration by parts formula is: u dv = uv - v du
Where u and dv are functions of x, and v is the antiderivative of dv.

Step 2: Identify u and dv
Choosing the right u and dv is crucial. Here are some tips:

• Choose u as the function that simplifies when differentiated
• Choose dv as the function that's easier to integrate
• Remember the LIATE rule: Logarithmic, Inverse trigonometric function, Algebraic, Trigonometric function, Exponential

Step 3: Find du and v
Once you've chosen u and dv:

• Differentiate u to get du
• Integrate dv to get v

Step 4: Apply the Formula
Substitute the values into the formula: u dv = uv - v du

Step 5: Solve the Resulting Integral
Evaluate v du. This may require further integration techniques or might simplify to a basic integral.

Example: Let's integrate x cos(x) dx

Step 1: Identify u and dv
u = x (algebraic, simplifies when differentiated)
dv = cos(x) dx (trigonometric, easier to integrate)

Step 2: Find du and v
du = dx (derivative of x)
v = sin(x) (integral of cos(x))

Step 3: Apply the Formula
x cos(x) dx = x sin(x) - sin(x) dx

Step 4: Solve the Resulting Integral
sin(x) dx = -cos(x)
So, x cos(x) dx = x sin(x) + cos(x) + C

Tips for Success:

• Practice identifying u and dv in various types of integrals
• Remember that the goal is to simplify the integral
• Sometimes, you may need to apply integration by parts multiple times

Integration by parts is a versatile method that can handle a wide range of complex integrals. By following this step-by-step guide and practicing with various examples, you'll become proficient in applying this technique. Remember, the key to success lies in choosing the right u and dv, and in recognizing when to use this method. With time and practice, you'll develop an intuition for tackling even the most challenging integrals using integration by parts.

## Special Cases in Integration by Parts

Integration by parts is a powerful technique in calculus, but it becomes even more versatile when applied to special cases. This section explores unique situations where integration by parts can be used creatively to solve complex integrals.

One special case involves integrals that don't appear to be products at first glance. A prime example is the integral of ln(x). While it may not look like a product, we can rewrite it as ln(x) · 1 to apply integration by parts. Let's walk through this process:

ln(x)dx = ln(x) · 1dx

Let u = ln(x) and dv = 1dx

Then du = 1/x dx and v = x

Applying the integration by parts formula:

ln(x)dx = x ln(x) - x · (1/x)dx = x ln(x) - x + C

This example demonstrates how integration by parts can be applied to functions that initially don't seem suitable for the technique. By creatively choosing u and dv, we can transform seemingly difficult integrals into manageable ones.

Another special case involves the need for multiple applications of integration by parts. Some integrals require repeated use of the technique to reach a solution. A classic example is the integral of x · e^x. Let's examine this step-by-step:

x · e^x dx

First application:

Let u = x and dv = e^x dx

Then du = dx and v = e^x

x · e^x dx = x · e^x - e^x dx

We're not done yet, as we still have e^x dx to solve. This integral is straightforward, but in more complex cases, we might need to apply integration by parts again.

x · e^x dx = x · e^x - e^x + C

A more challenging example that requires multiple applications is x^2 · sin(x)dx. This integral needs two rounds of integration by parts:

First application:

Let u = x^2 and dv = sin(x)dx

Then du = 2x dx and v = -cos(x)

x^2 · sin(x)dx = -x^2 · cos(x) + 2x · cos(x)dx

Second application (for 2x · cos(x)dx):

Let u = 2x and dv = cos(x)dx

Then du = 2dx and v = sin(x)

2x · cos(x)dx = 2x · sin(x) - 2 · sin(x)dx

Combining the results:

x^2 · sin(x)dx = -x^2 · cos(x) + 2x · sin(x) - 2 · (-cos(x)) + C

= -x^2 · cos(x) + 2x · sin(x) + 2cos(x) + C

These examples highlight the importance of recognizing when multiple applications of integration by parts are necessary. The key is to continue the process until you reach an integral that can be solved directly or until you notice a pattern that allows you to set up an equation to solve for the original integral.

Another special case worth mentioning is when the integral repeats itself after applying integration by parts. This situation often arises with trigonometric an

## Common Mistakes and Tips for Success

Integration by parts is a powerful technique in calculus, but students often encounter challenges when applying it. Understanding common mistakes and implementing effective strategies can significantly improve success rates. One frequent error is incorrectly identifying when to use integration by parts. Students should remember the LIATE rule (Logarithmic, Inverse trigonometric, Algebraic, Trigonometric, Exponential) to prioritize which function to choose as 'u' and which as 'dv'. Failing to follow this guideline can lead to unnecessarily complicated integrations or even dead ends.

Another common pitfall is forgetting to include the constant of integration in the final answer. This oversight can result in an incomplete solution. Students should always add '+C' at the end of their indefinite integrals to account for this constant. Additionally, errors in differentiation or integration of the chosen parts can derail the entire process. To avoid this, it's crucial to practice and reinforce basic differentiation and integration rules.

When it comes to successfully identifying when to use integration by parts, look for products of functions where one function becomes simpler when differentiated, and the other remains integrable when multiplied by x. This technique is particularly useful for integrals involving logarithms, inverse trigonometric functions, or products of polynomials with trigonometric, exponential, or logarithmic functions.

To avoid errors in the integration process, always clearly write out each step, including the choices for 'u' and 'dv'. This organized approach helps in tracking the progression and spotting potential mistakes. It's also beneficial to double-check each differentiation and integration step, as small errors can compound into significant problems.

Verifying the correctness of the solution is a critical final step. One effective strategy is to differentiate the final answer and compare it to the original integrand. If they match, it's a strong indication that the solution is correct. Another useful technique is to use a graphing calculator or computer algebra system to plot both the original function and the derivative of your solution. If the graphs align, it further confirms the accuracy of your work.

For more complex problems, consider breaking down the integral into smaller, manageable parts. This approach not only simplifies the process but also reduces the likelihood of errors. Finally, practice with a variety of problems to build confidence and intuition. The more diverse the problems you encounter, the better equipped you'll be to tackle challenging integrals using integration by parts.

## Conclusion and Further Practice

In this article, we've explored the powerful integration technique known as integration by parts. We've covered its formula, application, and step-by-step process. The introduction video serves as a crucial foundation for understanding this method, so be sure to watch it carefully. Remember, mastering integration by parts requires practice. Challenge yourself with various types of integrals to improve your skills and deepen your understanding. As you progress, you'll find this technique invaluable for solving complex integration problems. To further enhance your knowledge, we encourage you to attempt the practice integration problems provided or explore related videos on advanced calculus topics. Don't hesitate to revisit this article and the accompanying video as needed. With consistent practice and dedication, you'll soon become proficient in using integration by parts, opening doors to more advanced mathematical concepts. Keep pushing your boundaries and enjoy the journey of mathematical discovery!

### FAQs

1. What is integration by parts and when should I use it?

Integration by parts is a technique used to integrate products of functions. It's particularly useful when dealing with integrals involving products of polynomials with trigonometric, exponential, or logarithmic functions. Use it when one function in the product becomes simpler when differentiated, while the other remains manageable when integrated.

2. What is the formula for integration by parts?

The formula for integration by parts is: u dv = uv - v du. Here, u and dv are functions you choose from the integrand, v is the antiderivative of dv, and du is the derivative of u.

3. How do I choose u and dv in integration by parts?

Use the LIATE rule: choose u as the function that comes first in this order: Logarithmic, Inverse trigonometric, Algebraic, Trigonometric, Exponential. Choose dv as the remaining part of the integrand. Generally, u should be the function that simplifies when differentiated, and dv should be easily integrable.

4. Can integration by parts be applied multiple times to the same problem?

Yes, some integrals require multiple applications of integration by parts. This is common in problems involving higher-degree polynomials multiplied by trigonometric or exponential functions. Continue applying the technique until you reach a solvable integral or recognize a pattern that allows you to set up an equation for the original integral.

5. What are some common mistakes to avoid when using integration by parts?

Common mistakes include incorrectly choosing u and dv, forgetting the constant of integration, errors in differentiation or integration steps, and not recognizing when to stop applying the technique. Always verify your solution by differentiating your final answer and comparing it to the original integrand.

### Prerequisite Topics for Integration by Parts

Understanding integration by parts is a crucial skill in calculus, but to master this technique, it's essential to have a solid foundation in several prerequisite topics. These fundamental concepts not only prepare you for integration by parts but also enhance your overall mathematical prowess.

One of the key prerequisites is the power of a product rule, which is closely related to the product rule in differential calculus. This rule forms the basis for understanding how to break down complex integrals into simpler parts, a core principle of integration by parts.

Additionally, a strong grasp of polynomial and transcendental functions is crucial. These functions often appear in integration by parts problems, and knowing how to manipulate them is vital for successful problem-solving.

The derivative of exponential functions and the derivative of logarithmic functions are also essential. These concepts are frequently encountered in integration by parts, especially when dealing with natural logarithms or exponential expressions.

Understanding the derivative of inverse trigonometric functions is another crucial prerequisite. These functions often appear in complex integrals that require integration by parts, and knowing how to handle them is key to solving such problems effectively.

Trigonometric substitution is a related technique that complements integration by parts. While not always directly used in integration by parts, understanding this method can provide alternative approaches to solving certain integrals.

Lastly, familiarity with partial fractions in integration can be beneficial. Although it's a separate integration technique, it sometimes works in conjunction with integration by parts for solving complex rational functions.

By mastering these prerequisite topics, you'll build a strong foundation for tackling integration by parts. Each concept contributes to your overall understanding, allowing you to approach complex integrals with confidence and skill. Remember, in mathematics, each new concept builds upon previous knowledge, so investing time in these prerequisites will pay dividends as you advance in your calculus studies.

Integration by parts: $\smallint u{d}v = uv - \smallint v{d}u$
*strategy: choose u = f(x) to be a function that becomes simpler when differentiated.