Properties of Infinite Geometric Series

Oct 13, 2025 by wbrum.org 88 views

Introduction

In mathematics, an infinite geometric series is a type of series where each term is obtained by multiplying the previous term by a fixed constant. The series is said to be infinite because it has an infinite number of terms. In this article, we will explore the concept of infinite geometric series and how to find the sum of such a series.

What is an Infinite Geometric Series?

An infinite geometric series is a series of the form:

S = a + ar + ar^2 + ar^3 + \ldots

where $a$ is the first term and $r$ is the common ratio. The series is infinite because it has an infinite number of terms.

Properties of Infinite Geometric Series

For an infinite geometric series to converge, the common ratio $r$ must satisfy the condition $|r| < 1$ . If $|r| \geq 1$ , the series diverges.

Finding the Sum of an Infinite Geometric Series

If the series converges, the sum of the series can be found using the formula:

S = \frac{a}{1 - r}

where $a$ is the first term and $r$ is the common ratio.

Example: Finding the Sum of the Series $S=1+\frac{3}{4}+\frac{9}{16}+\frac{27}{64}+\ldots$

In this example, the first term $a = 1$ and the common ratio $r = \frac{3}{4}$ . To find the sum of the series, we can use the formula:

S = \frac{a}{1 - r}

Substituting the values of $a$ and $r$ , we get:

S = \frac{1}{1 - \frac{3}{4}}

Simplifying the expression, we get:

S = \frac{1}{\frac{1}{4}}

S = 4

Therefore, the sum of the series $S=1+\frac{3}{4}+\frac{9}{16}+\frac{27}{64}+\ldots$ is $S=4$ .

Conclusion

In this article, we have explored the concept of infinite geometric series and how to find the sum of such a series. We have also seen an example of how to find the sum of an infinite geometric series using the formula $S = \frac{a}{1 - r}$ . The sum of the series $S=1+\frac{3}{4}+\frac{9}{16}+\frac{27}{64}+\ldots$ is $S=4$ .

References

[1] "Infinite Geometric Series" by Math Open Reference
[2] "Infinite Geometric Series" by Khan Academy

Discussion

Do you have any questions about infinite geometric series or finding the sum of such a series? Please feel free to ask in the comments section below.

Infinite Arithmetic Series

An infinite arithmetic series is a series of the form:

S = a + (a + d) + (a + 2d) + (a + 3d) + \ldots

where $a$ is the first term and $d$ is the common difference.

Convergence of Infinite Series

An infinite series is said to converge if the sum of the series is finite. In other words, the series converges if the limit of the partial sums exists.

Divergence of Infinite Series

Q: What is an infinite geometric series?

A: An infinite geometric series is a type of series where each term is obtained by multiplying the previous term by a fixed constant. The series is said to be infinite because it has an infinite number of terms.

Q: What are the properties of an infinite geometric series?

A: For an infinite geometric series to converge, the common ratio $r$ must satisfy the condition $|r| < 1$ . If $|r| \geq 1$ , the series diverges.

Q: How do I find the sum of an infinite geometric series?

A: If the series converges, the sum of the series can be found using the formula:

S = \frac{a}{1 - r}

where $a$ is the first term and $r$ is the common ratio.

Q: What happens if the series diverges?

A: If the series diverges, the sum of the series does not exist. In other words, the limit of the partial sums does not exist.

Q: Can you give an example of an infinite geometric series?

A: Yes, the series $S=1+\frac{3}{4}+\frac{9}{16}+\frac{27}{64}+\ldots$ is an example of an infinite geometric series. The first term $a = 1$ and the common ratio $r = \frac{3}{4}$ .

Q: How do I find the sum of the series $S=1+\frac{3}{4}+\frac{9}{16}+\frac{27}{64}+\ldots$ ?

A: To find the sum of the series, we can use the formula:

S = \frac{a}{1 - r}

Substituting the values of $a$ and $r$ , we get:

S = \frac{1}{1 - \frac{3}{4}}

Simplifying the expression, we get:

S = \frac{1}{\frac{1}{4}}

S = 4

Therefore, the sum of the series $S=1+\frac{3}{4}+\frac{9}{16}+\frac{27}{64}+\ldots$ is $S=4$ .

Q: What is the difference between an infinite geometric series and an infinite arithmetic series?

A: An infinite geometric series is a series where each term is obtained by multiplying the previous term by a fixed constant, while an infinite arithmetic series is a series where each term is obtained by adding a fixed constant to the previous term.

Q: Can you give an example of an infinite arithmetic series?

A: Yes, the series $S=1+2+3+4+\ldots$ is an example of an infinite arithmetic series. The first term $a = 1$ and the common difference $d = 1$ .

Q: How do I find the sum of the series $S=1+2+3+4+\ldots$ ?

A: To find the sum of the series, we can use the formula:

S = \frac{n}{2} \cdot (a + l)

where $n$ is the number of terms, $a$ is the first term, and $l$ is the last term.

In this case, the number of terms $n$ is infinite, so the sum of the series does not exist.

Q: What is the difference between convergence and divergence of an infinite series?

A: Convergence of an infinite series means that the sum of the series is finite, while divergence of an infinite series means that the sum of the series is infinite.

Q: Can you give an example of a series that converges?

A: Yes, the series $S=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\ldots$ is an example of a series that converges. The first term $a = 1$ and the common ratio $r = \frac{1}{2}$ .

Q: How do I find the sum of the series $S=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\ldots$ ?

A: To find the sum of the series, we can use the formula:

S = \frac{a}{1 - r}

Substituting the values of $a$ and $r$ , we get:

S = \frac{1}{1 - \frac{1}{2}}

Simplifying the expression, we get:

S = \frac{1}{\frac{1}{2}}

S = 2

Therefore, the sum of the series $S=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\ldots$ is $S=2$ .

Q: Can you give an example of a series that diverges?

A: Yes, the series $S=1+2+4+8+\ldots$ is an example of a series that diverges. The first term $a = 1$ and the common ratio $r = 2$ .

Q: How do I find the sum of the series $S=1+2+4+8+\ldots$ ?

A: To find the sum of the series, we can use the formula:

S = \frac{a}{1 - r}

Substituting the values of $a$ and $r$ , we get:

S = \frac{1}{1 - 2}

Simplifying the expression, we get:

S = \frac{1}{-1}

S = -1

Therefore, the sum of the series $S=1+2+4+8+\ldots$ is $S=-1$ .

Note: The sum of the series $S=1+2+4+8+\ldots$ is actually infinite, but the formula $S = \frac{a}{1 - r}$ gives a finite value. This is because the formula is only valid for convergent series.