# Ratio test

##### Intros
###### Lessons
1. Ratio Test Overview
##### Examples
###### Lessons
1. Convergence & Divergence of Ratio Test
Use the Ratio Test to determine if the series converges or diverges. If the ratio test does not determine the convergence or divergence of the series, then resort to another test.
1. $\sum_{n=1}^{\infty}\frac{4^nn^3}{n!}$
2. $\sum_{n=0}^{\infty}\frac{(-2)^{2n}}{3^n(n+2)!}$
3. $\sum_{n=1}^{\infty}\frac{(2n+3)!}{n^2}$
4. $\sum_{n=0}^{\infty}\frac{3n+6}{n+2}$
2. Using the Ratio Test Twice to Show Convergence
Determine if the series $\sum_{k=1}^{\infty}\frac{4^k+k}{(k+1)!}$ converges or diverges.
###### Topic Notes
In this lesson, we will learn about the ratio test. This test requires you to calculate the value of R using the formula below. If R is greater than 1, then the series is divergent. If R is less than 1, then the series is convergent. If R is equal to 1, then the test fails and you would have to use another test to show the convergence or divergence of the series. It is really recommended to use this test if your series has factorials in it. We will do a few questions which involve using the ratio test, and then look at a question that requires us to use the ratio test twice.

## Introduction to the Ratio Test

The ratio test is a powerful tool in mathematical analysis for determining the convergence or divergence of infinite series. As introduced in our video, this method provides a straightforward approach to evaluating complex series. The test examines the limit of the ratio of consecutive terms in a series, offering insights into its behavior as it extends to infinity. By comparing this limit to 1, we can draw conclusions about the series' convergence. The ratio test is particularly useful for series involving factorials, exponentials, or powers, where other tests might be less effective. Understanding the ratio test is crucial for students and professionals dealing with advanced calculus and mathematical analysis. It forms a fundamental part of the toolkit for analyzing series convergence, alongside other methods like the comparison test and the root test. Mastering the ratio test opens doors to solving more complex problems in various mathematical and scientific fields.

## Understanding the Ratio Test Formula

The ratio test formula is a powerful tool in calculus used to determine the convergence or divergence of infinite series. This test is particularly useful when dealing with series that involve factorials, exponentials, or powers. Let's break down the formula and explore its components in detail.

The ratio test formula is expressed as:

R = lim(n) |a(n+1)/a(n)|

Here's what each part of the formula means:

• R: This represents the limit of the ratio between consecutive terms in the series.
• lim(n): This indicates that we're looking at the limit as n approaches infinity.
• a(n+1): This represents the (n+1)th term in the series.
• a(n): This represents the nth term in the series.
• | |: These vertical bars denote the absolute value of the ratio.

The significance of the limit in this formula cannot be overstated. By examining what happens to the ratio of consecutive terms as we move further along in the series (i.e., as n gets very large), we can predict the series' behavior in the long run. This limit gives us insight into whether the series will converge to a finite sum or diverge to infinity.

The absolute value in the formula is crucial because it allows us to consider the magnitude of the ratio without worrying about its sign. This is important because we're interested in how quickly the terms are growing or shrinking in size, regardless of whether they're positive or negative.

To use the ratio test, follow these steps:

1. Set up the ratio a(n+1)/a(n).
2. Simplify this ratio as much as possible.
3. Take the limit of this ratio as n approaches infinity.
4. Take the absolute value of this limit.
5. Compare the result (R) to 1:
• If R < 1, the series converges absolutely.
• If R > 1, the series diverges.
• If R = 1, the test is inconclusive, and you'll need to use other methods.

The beauty of the ratio test lies in its ability to handle complex series with relative ease. It's particularly effective for series involving factorials, exponentials, and powers, where other tests might be more cumbersome.

Remember, while the ratio test is powerful, it's not infallible. There are cases where it may be inconclusive (when R = 1), and in such situations, you'll need to employ other convergence tests or analytical methods.

In practice, mastering the ratio test formula requires patience and practice. Start with simple series and gradually work your way up to more complex ones. Pay close attention to the algebraic manipulations required to simplify the ratio a(n+1)/a(n) before taking the limit. With time and experience, you'll find the ratio test becoming an invaluable tool in your mathematical toolkit for analyzing infinite series.

## Interpreting Ratio Test Results

The ratio test is a powerful tool in calculus for determining the convergence or divergence of infinite series. Understanding the three possible outcomes of the ratio test is crucial for its proper application. Let's explore these outcomes in detail: R < 1 (convergence), R > 1 (divergence), and R = 1 (test fails).

### Case 1: R < 1 (Convergence)

When the limit of the ratio of consecutive terms is less than 1, the series converges. This means that as we add more terms, the sum approaches a finite value. For example, consider the series Σ(1/2^n) from n=1 to infinity. The ratio of consecutive terms is (1/2^(n+1)) / (1/2^n) = 1/2. Since 1/2 < 1, this series converges to 1.

### Case 2: R > 1 (Divergence)

If the limit of the ratio exceeds 1, the series diverges. In this case, the sum grows without bound as we add more terms. An example is the series Σ(2^n) from n=1 to infinity. The ratio of consecutive terms is 2^(n+1) / 2^n = 2. Since 2 > 1, this series diverges, growing exponentially.

### Case 3: R = 1 (Test Fails)

When the limit of the ratio equals 1, the ratio test is inconclusive. This doesn't mean the series neither converges nor diverges; it simply means we need to use other methods to determine its behavior. The harmonic series Σ(1/n) from n=1 to infinity is a classic example. The ratio of consecutive terms approaches 1, but the series actually diverges (proven by other tests).

### Importance of Understanding Ratio Test Conditions

Grasping these conditions is essential for several reasons:

• Accurate Analysis: Knowing when the ratio test converges or diverges allows for precise evaluation of series behavior.
• Efficient Problem-Solving: Recognizing the conditions quickly helps in choosing the most appropriate test for a given series.
• Avoiding Misinterpretation: Understanding that R = 1 doesn't guarantee convergence or divergence prevents incorrect conclusions.
• Application in Real-World Scenarios: Many scientific and engineering problems involve infinite series, making this knowledge practical beyond mathematics.

To apply the ratio test effectively, follow these steps:

1. Form the ratio of consecutive terms: a_(n+1) / a_n
2. Take the limit of this ratio as n approaches infinity
3. Compare the result to 1 and interpret based on the three conditions

Remember, while the ratio test is powerful, it's not always conclusive. When R = 1, or when the limit doesn't exist, you'll need to employ other convergence tests like the root test, comparison test, or integral test.

In conclusion, mastering the ratio test conditions enhances your ability to analyze infinite series. Whether you're dealing with power series, alternating series, or other types of infinite sums, understanding when the ratio test converges, diverges, or fails to provide a conclusion is a valuable skill in advanced calculus and mathematical analysis.

## When to Use the Ratio Test

The ratio test is a powerful tool in calculus for determining the convergence or divergence of infinite series. This test is particularly useful in specific scenarios, especially when dealing with complex series that involve factorials, exponentials, or powers. Understanding when to use the ratio test can significantly simplify the process of analyzing series convergence.

One of the primary scenarios where the ratio test shines is with series containing factorials. For instance, consider a series of the form Σ(n^n / n!). The presence of both powers and factorials makes this type of series challenging to evaluate using other convergence tests. However, the ratio test handles such series with remarkable efficiency. By comparing successive terms, it can quickly determine whether the series converges or diverges.

Another scenario where the ratio test is particularly useful is for series with exponential terms. For example, in a series like Σ(2^n / 3^n), the ratio test can easily compare the growth rates of the numerator and denominator. This comparison is crucial in determining whether the series will eventually converge or if it will grow unboundedly.

The ratio test is also recommended for series where the general term involves a combination of polynomial and exponential functions. For instance, in a series like Σ(n^2 / e^n), the interplay between the polynomial and exponential functions can be efficiently analyzed using the ratio test. This test can reveal whether the exponential growth in the denominator eventually overpowers the polynomial growth in the numerator.

It's worth noting that the ratio test is often the most efficient method for series where other tests, such as the comparison test or the integral test, might be cumbersome or inconclusive. For example, in series with complex terms like Σ((n^2 + 1)^n / (n^3 + 2)^n), the ratio test can provide a straightforward approach to determining convergence.

The ratio test is particularly effective for series where the terms involve products or quotients that become more complex as n increases. In such cases, taking the limit of the ratio of successive terms can simplify the analysis considerably. This is why the ratio test is often the go-to method for series involving factorials or powers with increasing complexity.

However, it's important to remember that while the ratio test is powerful, it's not always conclusive. In cases where the limit of the ratio equals 1, the test is inconclusive, and other methods must be employed. Despite this limitation, the ratio test remains a crucial tool in the arsenal of techniques for analyzing series convergence, especially for sequences with rapidly growing or decreasing terms.

## Step-by-Step Application of the Ratio Test

The ratio test is a powerful tool for determining the convergence or divergence of infinite series. This step-by-step guide will walk you through how to use the ratio test, complete with a practical example and tips for overcoming common challenges.

### Step 1: Identify the General Term

Begin by identifying the general term (an) of the series. For our example, let's use the series: Σ (n! / 3n).

### Step 2: Form the Ratio

Create the ratio of consecutive terms: |an+1 / an|. In our example:

|an+1 / an| = |(n+1)! / 3n+1| / |n! / 3n|

### Step 3: Simplify the Ratio

Simplify this expression as much as possible:

= ((n+1)! / 3n+1) / (n! / 3n)

= ((n+1) * n!) / (3 * 3n) * (3n / n!)

= (n+1) / 3

### Step 4: Calculate the Limit

Find the limit of this ratio as n approaches infinity:

limn (n+1) / 3 =

### Step 5: Interpret the Result

Apply the ratio test criteria:

• If the limit < 1, the series converges absolutely.
• If the limit > 1, the series diverges.
• If the limit = 1, the test is inconclusive.

In our example, the limit is infinity, which is greater than 1. Therefore, the series diverges.

### Tips for Handling Common Challenges

1. Complex General Terms: Break down complicated expressions into simpler parts before forming the ratio.
2. Factorials: When dealing with factorials, remember that (n+1)! = (n+1) * n!.
3. Exponentials: Use the properties of exponents to simplify ratios involving powers.
4. Trigonometric Functions: Be cautious with trigonometric terms; sometimes, other tests may be more suitable.
5. Inconclusive Results: If the ratio test is inconclusive (limit = 1), consider using other convergence tests like the root test or integral test.

### Practice Example

Let's apply the ratio test to another series: Σ (2n / n!)

1. General term: an = 2n / n!
2. Form the ratio: |an+1 / an| = |(2n+1 / (n+1)!) / (2n / n!)|
3. Simplify: = (2n+1 * n!) / ((n+1)! * 2n) = 2 / (n+1)

## Comparing Ratio Test with Other Convergence Tests

The ratio test is a powerful tool for determining the convergence of infinite series, but it's essential to understand how it compares to other convergence tests and when to use each. Let's explore the relationship between the ratio test and other common tests, such as the divergence test and alternating harmonic series, while also discussing scenarios where the ratio test may fall short.

The ratio test is particularly effective for series involving factorials, exponentials, or powers. It works by examining the limit of the ratio of consecutive terms in a series. When this limit is less than 1, the series converges; when it's greater than 1, the series diverges. However, the ratio test has limitations, especially when the limit equals 1.

In contrast, the divergence test, also known as the nth term test, is simpler but less conclusive. It states that if the limit of the general term of a series is not zero, the series diverges. While this test can quickly identify some divergent series, it cannot confirm convergence when the limit is zero.

The alternating harmonic series test, on the other hand, is specifically designed for series where terms alternate between positive and negative. It provides conditions for convergence based on the absolute value of terms decreasing and approaching zero. This test is particularly useful for series that the ratio test might struggle with, such as alternating harmonic series.

When deciding which test to use, consider the following guidelines:

1. Use the ratio test when dealing with series involving factorials, exponentials, or powers. It's especially effective for series where terms have a consistent pattern of growth or decay.

2. Apply the divergence test as a quick initial check. If the limit of the general term is not zero, you can immediately conclude divergence without needing more complex tests.

3. For alternating series, start with the alternating harmonic series test. It's specifically designed for these types of series and can provide conclusive results where the ratio test might be inconclusive.

4. If the ratio test yields a limit of 1, it's inconclusive, and you should proceed with alternative tests. In such cases, consider using the root test, integral test, or comparison tests.

It's important to note that when the ratio test fails (R = 1), it doesn't mean the series necessarily converges or diverges; it simply means the test is inconclusive for that particular series. In these scenarios, you might need to employ multiple tests or more advanced techniques to determine convergence.

For example, consider the harmonic series 1/n. The ratio test yields a limit of 1, making it inconclusive. In this case, you could use the integral test or direct comparison test to prove divergence. Similarly, for the alternating harmonic series (-1)^(n+1)/n, the ratio test is again inconclusive, but the alternating series test easily proves convergence.

In conclusion, while the ratio test is a valuable tool in the arsenal of convergence tests, it's crucial to be familiar with a range of tests and understand their strengths and limitations. By knowing when to apply each test and how to proceed when one test is inconclusive, you can effectively analyze a wide variety of infinite series. Remember, in many cases, a combination of tests may be necessary to reach a definitive conclusion about a series' convergence or divergence.

## Practice Problems and Solutions

Welcome to our practice section on the ratio test! Below, you'll find a set of carefully selected problems that demonstrate various applications of this powerful convergence test. We've included examples where the ratio test succeeds as well as cases where it fails, to give you a comprehensive understanding. Before diving into the solutions, we encourage you to attempt these problems on your own. This approach will help reinforce your learning and improve your problem-solving skills.

### Problem 1: Basic Application

Determine the convergence of the series: (n^2 / 3^n) from n=1 to infinity

#### Solution:

Step 1: Apply the ratio test. Let a_n = n^2 / 3^n
Step 2: Calculate the limit of |a_(n+1) / a_n| as n approaches infinity
lim(n) |(n+1)^2 / 3^(n+1)| / |n^2 / 3^n|
= lim(n) |(n+1)^2 / n^2| * |3^n / 3^(n+1)|
= lim(n) (n+1)^2 / (3n^2)
= lim(n) (n^2 + 2n + 1) / (3n^2)
= lim(n) (1 + 2/n + 1/n^2) / 3
= 1/3
Step 3: Since the limit is 1/3 < 1, the series converges.

### Problem 2: Ratio Test Failure

Examine the convergence of the series: (1/n) from n=1 to infinity

#### Solution:

Step 1: Apply the ratio test. Let a_n = 1/n
Step 2: Calculate the limit of |a_(n+1) / a_n| as n approaches infinity
lim(n) |1/(n+1)| / |1/n|
= lim(n) n / (n+1)
= 1
Step 3: Since the limit equals 1, the ratio test is inconclusive. We need to use another test (in this case, the integral test would show that this is the divergent harmonic series).

### Problem 3: Complex Ratio

Determine if the series ((2n)! / (n! * 4^n)) converges or diverges.

#### Solution:

Step 1: Apply the ratio test. Let a_n = (2n)! / (n! * 4^n)
Step 2: Calculate the limit of |a_(n+1) / a_n| as n approaches infinity
lim(n) |(2n+2)! / ((n+1)! * 4^(n+1))| / |(2n)! / (n! * 4^n)|
= lim(n) ((2n+2)(2n+1) * (2n)!) / ((n+1) * n! * 4^(n+1)) * (n! * 4^n) / (2n)!
= lim(n) (2n+2)(2n+1) / (4(n+1))
= lim(n) (4n^2 + 6n + 2) / (4n + 4)
= lim(n) (n^2 + 3n/2 + 1/

## Conclusion

The ratio test is a powerful tool for determining series convergence, offering a straightforward approach to evaluating complex series. By examining the limit of the ratio of consecutive terms, we can quickly assess whether a series converges or diverges. This test is particularly useful for series involving factorials, exponentials, and certain polynomial expressions. Remember, if the limit is less than 1, the series converges; if greater than 1, it diverges; and if equal to 1, the test is inconclusive. The introduction video provided a solid foundation for understanding the ratio test's application and significance. To truly master this concept, practice is key. Explore various series types, work through additional problems, and consider how the ratio test relates to other convergence tests. By honing your skills with the ratio test, you'll strengthen your overall understanding of series convergence and mathematical analysis. Keep practicing and expanding your knowledge in this crucial area of calculus.

### Example:

Convergence & Divergence of Ratio Test
Use the Ratio Test to determine if the series converges or diverges. If the ratio test does not determine the convergence or divergence of the series, then resort to another test.
$\sum_{n=1}^{\infty}\frac{4^nn^3}{n!}$

#### Step 1: Identify $a_n$ and $a_{n+1}$

To apply the Ratio Test, we first need to identify the general term $a_n$ of the series and then find $a_{n+1}$. For the given series $\sum_{n=1}^{\infty}\frac{4^nn^3}{n!}$, we have: $a_n = \frac{4^n n^3}{n!}$ Next, we need to find $a_{n+1}$, which is the term when $n$ is replaced by $n+1$: $a_{n+1} = \frac{4^{n+1} (n+1)^3}{(n+1)!}$

#### Step 2: Set Up the Ratio Test

The Ratio Test involves taking the limit of the absolute value of the ratio of consecutive terms. Specifically, we need to compute: $\lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right|$ Substituting $a_{n+1}$ and $a_n$ into this formula, we get: $\lim_{n \to \infty} \left| \frac{4^{n+1} (n+1)^3}{(n+1)!} \cdot \frac{n!}{4^n n^3} \right|$

#### Step 5: Conclusion from the Ratio Test

According to the Ratio Test, if the limit $L$ is less than 1, the series converges. In this case, we found that: $L = 0$ Since 0 < 1, the series $\sum_{n=1}^{\infty}\frac{4^nn^3}{n!}$ converges by the Ratio Test.

### FAQs

#### 1. What is the ratio test used to determine?

The ratio test is used to determine the convergence or divergence of infinite series. It examines the limit of the ratio of consecutive terms in a series to draw conclusions about its behavior.

#### 2. How do you apply the ratio test?

To apply the ratio test, follow these steps: 1. Form the ratio of consecutive terms: |a_(n+1) / a_n| 2. Take the limit of this ratio as n approaches infinity 3. If the limit is less than 1, the series converges; if greater than 1, it diverges; if equal to 1, the test is inconclusive

#### 3. When is the ratio test most effective?

The ratio test is particularly effective for series involving factorials, exponentials, or powers. It's often the go-to method for series where terms have a consistent pattern of growth or decay.

#### 4. Can the ratio test be used to find the radius of convergence?

Yes, the ratio test can be used to find the radius of convergence for power series. The radius of convergence is the reciprocal of the limit found in the ratio test, provided this limit exists and is not zero.

#### 5. What are the limitations of the ratio test?

The main limitation of the ratio test is that it's inconclusive when the limit equals 1. In such cases, other convergence tests must be used. Additionally, the ratio test doesn't provide information about conditional convergence; it only determines absolute convergence or divergence.

### Prerequisite Topics for Understanding the Ratio Test

When delving into the world of calculus, particularly the study of series convergence, the Ratio Test emerges as a powerful tool. However, to fully grasp its significance and application, it's crucial to have a solid foundation in several prerequisite topics. Understanding these fundamental concepts not only enhances your comprehension of the Ratio Test but also provides a broader perspective on series analysis in calculus.

One of the most critical prerequisites is convergence and divergence of infinite series. This concept forms the backbone of series analysis, providing the essential framework for determining whether a series has a finite sum or diverges to infinity. The Ratio Test builds upon this knowledge, offering a specific method to evaluate convergence for certain types of series.

Another important prerequisite is the derivative of exponential functions. While it might not seem immediately related, understanding how to differentiate exponential functions is crucial when applying the Ratio Test to series involving exponential terms. This knowledge allows you to manipulate and simplify complex expressions that often arise in ratio calculations.

The concept of harmonic series divergence is also highly relevant. Studying the behavior of harmonic series provides insights into the limitations of certain convergence tests and helps appreciate the power and versatility of the Ratio Test. It illustrates why we need various tests for different types of series and how the Ratio Test fits into this broader toolkit.

Lastly, familiarity with the Alternating Series Test is beneficial. While the Ratio Test is applicable to both alternating and non-alternating series, understanding the Alternating Series Test provides a comparative perspective. It helps you recognize when to apply the Ratio Test and when other methods might be more appropriate or efficient.

By mastering these prerequisite topics, you'll be well-equipped to tackle the Ratio Test with confidence. You'll understand not just how to apply the test mechanically, but also why it works and when it's most useful. This comprehensive understanding is invaluable in advanced calculus and forms a solid foundation for exploring more complex topics in mathematical analysis.

Remember, in mathematics, each new concept builds upon previous knowledge. The Ratio Test is no exception. By investing time in these prerequisite topics, you're not just preparing for one test; you're developing a robust mathematical toolkit that will serve you well throughout your studies and beyond. Embrace these foundational concepts, and you'll find that the Ratio Test becomes not just a formula to memorize, but a logical and powerful tool in your mathematical arsenal.

Note *Let $\sum a_n$ be a series. Then we say that

$R=$$\lim$n →$\infty$ $\mid \frac{a_{n+1}}{a_n}\mid$

Where:
1. If $R$ < $1$, then the series is convergent (or absolutely convergent)
2. If $R$ > $1$, then the series is divergent
3. If $R=1$, then the series could either be divergent, or convergent

Basically if $R=1$, then the ratio test fails and would require a different test to determine the convergence or divergence of the series.