# Infinite geometric series

##### Intros

###### Lessons

##### Examples

###### Lessons

**Using the common ratio to determine whether a sum to infinity exists**

For each geometric series determine the:

i) common ratio.

ii) sum of the first 10 terms.

iii) sum to infinity.

**Expressing a repeating decimal as an infinite geometric series**

For the repeating decimal:

i) express it as an infinite geometric series.

ii) write it as a fraction by evaluating the sum of the infinite geometric series.

##### Practice

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###### Topic Notes

## Introduction to Infinite Geometric Series

Infinite geometric series are a fascinating concept in mathematics, representing the sum of an endless sequence of numbers with a constant ratio between consecutive terms. Our introduction video provides a visual and intuitive understanding of this concept, serving as a crucial foundation for further exploration. In this article, we'll delve into the world of infinite geometric series, focusing on two key types: convergent series and divergent series. Convergent series approach a finite sum as the number of terms increases indefinitely, while divergent series do not. We'll explore the conditions that determine whether a series converges or diverges, and learn how to calculate the sum of a convergent infinite geometric series using a simple formula. Understanding these concepts is essential for advanced mathematics and has practical applications in fields such as physics, engineering, and finance. Join us as we unravel the mysteries of infinite geometric series and their powerful mathematical properties.

## Understanding Convergent Infinite Geometric Series

Let's dive into the fascinating world of convergent infinite geometric series, using a relatable example to make this concept crystal clear. Imagine you have a special machine that gives you money, but with a twist. On day one, it gives you $1000. The next day, it gives you $100, then $10, then $1, then $0.10, and so on. This sequence forms what we call an infinite geometric series.

Now, you might wonder, "If this goes on forever, how much money will I end up with?" This is where the magic of convergent series comes into play. In our example, the series looks like this: 1000 + 100 + 10 + 1 + 0.1 + ...

To understand why this series converges (reaches a finite sum) and how to calculate that sum, we need to introduce a crucial concept: the common ratio. In a geometric series, each term is a constant multiple of the previous term. This constant is called the common ratio. In our money machine example, each term is 1/10 of the previous term, so our common ratio is 0.1.

The common ratio plays a pivotal role in determining whether an infinite geometric series converges or diverges (grows without bound). Here's the key: if the absolute value of the common ratio is less than 1, the series converges. In our case, |0.1| < 1, so our series converges.

But why does a series with a common ratio between -1 and 1 converge? Think of it this way: when the common ratio is a fraction (positive or negative), each term becomes significantly smaller than the last. Eventually, the terms become so tiny that adding more of them doesn't meaningfully change the sum.

Let's break it down further. In our example:

- First term (a) = 1000
- Common ratio (r) = 0.1

For a convergent geometric series, we can use a formula to find the sum: S = a / (1 - r), where 'S' is the sum, 'a' is the first term, and 'r' is the common ratio.

Plugging in our values: S = 1000 / (1 - 0.1) = 1000 / 0.9 1111.11

So, despite this series going on infinitely, it converges to a sum of about $1111.11. Isn't that remarkable? No matter how long you wait, your total will never exceed this amount, but it will get closer and closer to it.

This concept extends beyond just positive fractions. Any common ratio between -1 and 1 (exclusive) leads to convergence. For instance, if our machine alternated between giving and taking money, with a ratio of -0.5, the series would still converge, just in a zigzag pattern.

Understanding convergent infinite geometric series is crucial in many areas of mathematics and real-world applications. It helps us make sense of situations where we have a diminishing effect over time or space, like calculating the total distance a bouncing ball travels or determining the present value of an annuity in finance.

Remember, the key to identifying a convergent infinite geometric series is the common ratio. If it's between -1 and 1 (but not equal to -1 or 1), you're dealing with a convergent series. This powerful concept allows us to sum an infinite number of terms to a finite, calculable value, opening up a world of mathematical possibilities.

## The Intuition Behind Convergent Series

Have you ever wondered how we can add up an infinite number of terms and still get a finite result? This fascinating concept is at the heart of convergent series, a key topic in mathematics that often puzzles students. Let's dive into the intuition behind this idea, using some friendly examples to make it more digestible.

Imagine you're adding up a list of numbers that goes on forever. At first, this might seem impossible - how can we sum an infinite number of terms? The key lies in understanding that in a convergent series, the terms we're adding become smaller and smaller as we go along. Eventually, these terms become so tiny that adding them makes virtually no difference to our total sum.

Think of it like this: you're building a tower with blocks. The first few blocks add significant height, but as you keep stacking, you start using thinner and thinner blocks. At some point, the blocks become so thin that adding more doesn't noticeably change the tower's height. This is similar to how convergent series work.

Let's consider a concrete example: the geometric series example 1 + 1/2 + 1/4 + 1/8 + 1/16 + ... Each term is half of the previous one. As we add these terms, we get closer and closer to 2, but we never quite reach it. The additional terms become so small that they're essentially adding zeros after a certain decimal place.

For instance, after adding the first few terms, we might have 1.9375. The next term, 1/32, only changes this to 1.96875. Subsequent terms add even smaller amounts. It's like adding 0.00... with more and more zeros before a tiny digit at the end. These additions become negligible, which is why we can say the series converges to 2, even though we're technically adding an infinite number of terms.

This concept of "adding zeros" after a certain point is crucial to understanding convergence. In our digital age, we often work with finite precision. On a calculator or computer, there's a limit to how many decimal places we can represent. Beyond this limit, adding more terms is like adding zeros - it doesn't change the displayed result.

The beauty of convergent series lies in this property: even though we're dealing with infinity, we end up with a finite, manageable result. It's a bit like zooming in on a fractal - no matter how far you zoom, you never reach the "end," but you can describe the overall shape and size.

Understanding convergent series opens up a world of mathematical possibilities. It allows us to work with infinite processes in a finite way, which is incredibly useful in calculus, physics, and many areas of science and engineering. From calculating compound interest to modeling decay processes, convergent series pop up everywhere.

So, the next time you encounter a geometric series example, remember this intuition. It's not about actually adding up an infinite number of terms - it's about recognizing that after a certain point, the additional terms become so small that they effectively don't matter. This is the magic of convergence, turning the infinite into something we can grasp and use in the real world.

## Divergent Infinite Geometric Series

Let's dive into the fascinating world of divergent infinite geometric series example, using the example from our video: 2 + 4 + 8 + 16 + ... This series might seem simple at first glance, but it holds the key to understanding a crucial concept in mathematics.

In a geometric series example, each term is a constant multiple of the previous term. This constant is called the common ratio. In our example, you can see that each term is twice the previous one, so the common ratio is 2. Now, what makes this series special and divergent?

A divergent series is one that doesn't converge to a finite sum. In other words, as we keep adding terms, the sum keeps growing without bound. This is exactly what happens with our example series. Let's break it down:

2 + 4 = 6

2 + 4 + 8 = 14

2 + 4 + 8 + 16 = 30

And so on...

As we continue this process, the sum keeps getting larger and larger, never settling on a fixed value. This is the hallmark of a divergent series.

But what causes a series to diverge? The key lies in the common ratio. For an infinite geometric series to converge, the common ratio must be between -1 and 1, not including -1 and 1 themselves. In our example, the common ratio is 2, which falls outside this range.

When the common ratio is greater than or equal to 1 (or less than or equal to -1), each term in the series is either equal to or greater in absolute value than the previous term. This means the series will keep growing indefinitely, resulting in divergence.

In mathematical terms, we say that the sum of a divergent series is undefined or approaches infinity. It's important to note that "infinity" in this context isn't a number we can reach, but rather a concept indicating that the series grows without bound.

Understanding divergent series is crucial in many areas of mathematics and its applications. They help us model situations where growth is unbounded, such as in certain economic scenarios or population models under ideal conditions.

To summarize, a divergent infinite geometric series, like our example 2 + 4 + 8 + 16 + ..., is characterized by:

- A common ratio outside the range of -1 to 1
- A sum that grows without bound as more terms are added
- An undefined or infinite result when we try to calculate the sum of all terms

Remember, while convergent series give us neat, finite sums, divergent series are equally important in mathematics. They challenge our understanding of infinity and help us explore the limits of mathematical concepts. So the next time you encounter a series that seems to grow endlessly, you'll know you're dealing with the intriguing world of divergent series!

## Determining Convergence or Divergence

Welcome to our guide on how to evaluate infinite geometric series! One of the most crucial skills in this area is determining whether a series converges or diverges. This knowledge is essential for finding the sum of an infinite series, so let's dive in and learn how to master this concept.

The key to determining convergence or divergence lies in the common ratio of the geometric series. The common ratio, usually denoted as 'r', is the number you multiply each term by to get the next term. Here's the golden rule: an infinite geometric series converges if and only if the absolute value of the common ratio is less than 1 (|r| < 1).

Let's break this down with some examples. Consider the series 1 + 1/2 + 1/4 + 1/8 + ... In this case, each term is half of the previous term, so our common ratio is 1/2. Since |1/2| = 0.5, which is less than 1, this series converges. On the other hand, take the series 1 + 2 + 4 + 8 + ... Here, each term is doubled, so the common ratio is 2. Since |2| > 1, this series diverges.

Now, what does convergence or divergence mean for finding the sum of the series? If a series converges, it means the sum of all its terms approaches a finite value. For our converging example above, we can actually calculate this sum using the formula S = a / (1 - r), where 'a' is the first term. So, 1 / (1 - 1/2) = 2. The series sum approaches 2 as we add more and more terms.

However, if a series diverges, like our second example, the sum grows without bound. There's no finite value that the sum approaches, no matter how many terms we add. This is why we can't use the sum formula for divergent series - it simply doesn't work!

When you're evaluating infinite geometric series, always start by identifying the common ratio. Once you have that, the convergence test is straightforward: just check if |r| < 1. If it is, you're dealing with a convergent series and can find its sum. If not, the series diverges and doesn't have a finite sum.

Remember, practice makes perfect! Try applying this convergence test to different series you encounter. Soon, you'll be able to spot convergent and divergent series at a glance. Keep exploring, and don't hesitate to dive deeper into the fascinating world of infinite series!

## Formula for the Sum of a Convergent Infinite Geometric Series

Welcome, math enthusiasts! Today, we're diving into the fascinating world of infinite series calculation and learning how to find their sum. Don't worry if it sounds intimidating we'll break it down step by step!

First, let's introduce the formula for the sum of a convergent infinite geometric series:

**S = a / (1 - r)**

Where:

- S is the sum of the infinite series
- a is the first term of the series
- r is the common ratio between terms

Now, let's break down each component:

**a (first term):**This is where our series begins. It's the starting point of our journey.**r (common ratio):**This is the multiplier that takes us from one term to the next. It's crucial for determining if the series converges.**1 - r:**This part of the formula is key to finding the sum. It's related to how the series behaves as it extends to infinity.

For a geometric series to converge, the absolute value of common ratio must be less than 1 (|r| < 1). This condition ensures that the terms get smaller and smaller, eventually approaching zero.

Let's walk through an example to see how this works in practice:

**Example 1:** Find the sum of geometric series example 8 + 4 + 2 + 1 + ...

- Identify a and r:
- a = 8 (first term)
- r = 4/8 = 1/2 (ratio between terms)

- Check if |r| < 1:
- |1/2| = 0.5, which is less than 1, so the series converges

- Apply the formula:
- S = a / (1 - r)
- S = 8 / (1 - 1/2)
- S = 8 / (1/2)
- S = 16

The sum of this infinite series is 16!

Let's try another example:

**Example 2:** Find the sum of geometric series example 3 - 1 + 1/3 - 1/9 + ...

- Identify a and r:
- a = 3 (first term)
- r = -1/3 (ratio between terms)

- Check if |r| < 1:
- |-1/3| = 1/3, which is less than 1, so the series converges

- Apply the formula:
- S = a / (1 - r)
- S = 3 / (1 - (-1/3))
- S = 3 / (4/3)
- S = 9/4 = 2.25

The sum of this infinite series is 2.25!

Now it's your turn to practice infinite series calculation and master the concepts!

## Applications of Infinite Geometric Series

Infinite geometric series might sound like a purely mathematical concept, but they have fascinating real-world applications that touch our lives in surprising ways. Let's explore how these series help us understand and solve practical problems across various fields.

In finance, infinite geometric series play a crucial role in calculating compound interest calculation. Imagine you've invested $1,000 in a savings account with an annual interest rate of 5%. Each year, your money grows by 5% of the previous year's amount. This creates a geometric series that theoretically continues indefinitely. Understanding this concept helps financial advisors and investors make informed decisions about long-term investments and retirement planning.

Physics also benefits from infinite geometric series, particularly in the study of radioactive decay half-life. When a radioactive substance decays, it loses half of its mass in a fixed period, known as its half-life. This process continues indefinitely, creating a geometric series that approaches zero but never quite reaches it. Scientists use this knowledge to date ancient artifacts, understand nuclear reactions, and develop medical imaging techniques.

Computer science and graphics rely heavily on infinite geometric series, especially in the creation of fractals. These intricate, self-repeating patterns are found in nature and are used in computer-generated imagery for films and video games. The Sierpinski triangle, for example, is formed by repeatedly removing smaller triangles from a larger one, creating a pattern that theoretically continues forever. This concept helps in developing efficient algorithms for data compression and computer graphics.

In telecommunications, infinite geometric series help engineers design signal processing systems. When signals bounce between two points, they create a series of echoes that diminish over time. This phenomenon can be modeled using an infinite geometric series, allowing engineers to develop better noise cancellation and echo suppression technologies for clearer communications.

Even in everyday scenarios, we encounter applications of infinite geometric series. Consider bouncing a ball: each bounce reaches a fraction of the previous height, creating a series that theoretically continues infinitely (though in reality, the ball eventually stops due to energy loss). Understanding this helps in designing sports equipment and predicting ball behavior in various games.

By grasping the concept of infinite geometric series, we gain powerful tools for problem-solving across diverse fields. Whether you're calculating the long-term value of an investment, predicting the decay of radioactive materials, or designing complex computer graphics, these mathematical concepts provide a foundation for understanding and manipulating real-world phenomena. The next time you see a fractal pattern or consider a long-term savings plan, remember the infinite geometric series working behind the scenes, helping us make sense of our complex and fascinating world.

## Conclusion

In this article, we explored the fascinating world of infinite geometric series, focusing on the crucial distinction between convergent and divergent series. We learned that convergent series approach a finite sum, while divergent series do not. To determine convergence, we examined the common ratio and absolute value criteria. For convergent series, we discovered the formula to calculate their sum. The introduction video proved invaluable in visualizing these abstract concepts, making them more accessible. Remember, convergent series are essential in various mathematical applications, from calculus to physics. To solidify your understanding, practice with additional examples and explore more advanced topics related to infinite geometric series. Don't hesitate to revisit the video for a refresher on the visual representations. By mastering these concepts, you'll unlock a powerful tool in mathematical analysis and problem-solving. Continue your journey into the realm of infinite series, and discover how they shape our understanding of mathematics and the world around us.

### Example:

**Using the common ratio to determine whether a sum to infinity exists**

For each geometric series determine the:

i) common ratio.

ii) sum of the first 10 terms.

iii) sum to infinity.

4 + 2 + 1 +

#### Step 1: Determine the Common Ratio

To find the common ratio of a geometric series, we need to take any two consecutive terms and divide the second term by the first term. For the given series 4 + 2 + 1 + , we can take the first two terms:

- First term (T1) = 4
- Second term (T2) = 2

The common ratio (R) is calculated as follows:

R = T2 / T1 = 2 / 4 = 1/2

To verify, we can also take the next pair of terms:

- Second term (T2) = 2
- Third term (T3) = 1

R = T3 / T2 = 1 / 2 = 1/2

Thus, the common ratio is confirmed to be 1/2.

#### Step 2: Check for Convergence

For a geometric series to have a sum to infinity, the common ratio must be between -1 and 1 (exclusive). Since our common ratio is 1/2, which lies between -1 and 1, the series is convergent. Therefore, a sum to infinity exists for this series.

#### Step 3: Calculate the Sum of the First 10 Terms

To find the sum of the first 10 terms of a geometric series, we use the formula for the sum of the first n terms (Sn):

Sn = T1 * (1 - R^n) / (1 - R)

For our series, T1 = 4, R = 1/2, and n = 10. Plugging these values into the formula, we get:

S10 = 4 * (1 - (1/2)^10) / (1 - 1/2)

Using a calculator to compute the values:

S10 = 4 * (1 - 1/1024) / (1/2)

S10 = 4 * (1023/1024) / (1/2)

S10 = 4 * 1023/512

S10 7.9922

Thus, the sum of the first 10 terms is approximately 7.9922.

#### Step 4: Calculate the Sum to Infinity

To find the sum to infinity of a convergent geometric series, we use the formula:

S = T1 / (1 - R)

For our series, T1 = 4 and R = 1/2. Plugging these values into the formula, we get:

S = 4 / (1 - 1/2)

S = 4 / (1/2)

S = 4 * 2

S = 8

Thus, the sum to infinity for the given series is 8.

#### Conclusion

In summary, for the geometric series 4 + 2 + 1 + :

- The common ratio is 1/2.
- The sum of the first 10 terms is approximately 7.9922.
- The sum to infinity is 8.

### FAQs

Here are some frequently asked questions about infinite geometric series:

#### 1. What is the formula for evaluating infinite geometric series?

The formula for the sum of a convergent infinite geometric series is S = a / (1 - r), where 'S' is the sum, 'a' is the first term, and 'r' is the common ratio. This formula is valid only when |r| < 1.

#### 2. How do you evaluate a geometric series?

To evaluate a geometric series, first identify the first term (a) and the common ratio (r). Then, determine if the series is finite or infinite. For finite series, use the formula S = a(1 - r^n) / (1 - r), where n is the number of terms. For infinite series, use S = a / (1 - r) if |r| < 1; otherwise, the series diverges.

#### 3. How do you prove a geometric series to infinity?

To prove a geometric series converges to infinity, show that the absolute value of the common ratio is less than 1 (|r| < 1). Then, demonstrate that the partial sums approach a limit as the number of terms increases. This can be done using the limit of the partial sums formula or by showing that the remainder term approaches zero.

#### 4. What is the intuition behind the infinite geometric series formula?

The intuition behind the formula S = a / (1 - r) comes from the idea of a "geometric sum trick." Multiply both sides by (1 - r), and you'll see that most terms cancel out, leaving only the first term. This algebraic manipulation reveals why the formula works and provides insight into the series' behavior.

#### 5. How do you determine if an infinite geometric series converges or diverges?

To determine convergence or divergence, examine the common ratio (r). If |r| < 1, the series converges, and you can use the sum formula. If |r| 1, the series diverges, and there is no finite sum. This criterion is based on whether the terms approach zero as you move further in the sequence.

### Prerequisite Topics for Infinite Geometric Series

Understanding infinite geometric series requires a solid foundation in several key mathematical concepts. One crucial prerequisite is absolute value functions. These functions play a vital role in determining the convergence of infinite geometric series. When dealing with complex series, the absolute value helps assess the magnitude of terms, which is essential for understanding the series' behavior as it extends infinitely.

Another important prerequisite topic is compound interest calculation. This concept shares striking similarities with infinite geometric series, as both involve repeated multiplication of a constant factor. In compound interest, this factor is (1 + r), where r is the interest rate. Understanding how compound interest grows over time provides valuable insights into the behavior of geometric series, especially when the common ratio is greater than 1.

The study of radioactive decay half-life also offers relevant background knowledge for infinite geometric series. This concept demonstrates a real-world application of geometric sequences, where the amount of a radioactive substance decreases by a constant factor over regular time intervals. This decay process mirrors the behavior of a geometric series with a common ratio between 0 and 1, providing a tangible example of how these series converge.

Mastering these prerequisite topics is crucial for a comprehensive understanding of infinite geometric series. The concept of absolute value helps in analyzing the convergence criteria, particularly when dealing with complex or alternating series. Compound interest calculations provide a practical context for understanding growth patterns in geometric sequences, which directly translates to the behavior of their corresponding series. Similarly, the study of radioactive decay offers insights into convergent series with ratios less than 1, illustrating how these series approach a finite sum despite having infinite terms.

By thoroughly grasping these prerequisite concepts, students can more easily navigate the complexities of infinite geometric series. They will be better equipped to understand the conditions for convergence, calculate sums of infinite series, and apply these concepts to real-world scenarios. Moreover, this foundational knowledge facilitates a deeper appreciation of the interconnectedness of mathematical concepts, enhancing overall mathematical reasoning and problem-solving skills.

In conclusion, while infinite geometric series may seem challenging at first, a solid understanding of absolute value functions, compound interest calculations, and exponential decay processes provides a robust framework for tackling this advanced topic. These prerequisites not only make learning infinite geometric series more accessible but also enrich the learning experience by connecting abstract mathematical concepts to practical applications.

###### Basic Concepts

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