# Sequences and Series

## The Distinction between Sequences and Series

Although sequences and series may seem similar, they are actually separate concepts. It's important to understand the differences between the two.

## Sequences

A sequence is a set of terms, usually numbers, that follow a specific rule. These terms are arranged in a specific order, typically based on a mathematical pattern. Some examples of sequences and their corresponding rules include:

• (3, 9, 15, 21, 27, 33) - Increasing by 6
• (72, 64, 56, 48, 40, 32) - Decreasing by 8
• (5, 10, 20, 40, 80, 160) - Multiplying by 2

Sequences can be either finite, like the ones shown above, or infinite, meaning they continue indefinitely. An infinite sequence can be represented as:

• (1, 2, 3, 4, 5, 6, ...)
• (4, 7, 10, 13, 16, ...)

Since infinite sequences have no end, we can use mathematical formulas to find specific terms instead of listing them all out. In this article, we will explore these formulas further.

### The Types of Sequences

There are two main types of sequences:

• Arithmetic sequences - where terms increase or decrease by a constant amount through addition or subtraction. This constant difference is known as the common difference, denoted as 'd'.
• Geometric sequences - where terms increase or decrease by a constant ratio through multiplication or division. This constant difference is known as the common ratio, denoted as 'r'.

## Series

A series is the sum of terms within a sequence. For example:

• (3, 9, 15, 21, 27, 33) is a sequence, and its series is 3 + 9 + 15 + 21 + 27 + 33
• (72, 64, 56, 48, 40, 32) is a sequence, and its series is 72 + 64 + 56 + 48 + 40 + 32

### Formulas for Sequences and Series

When working with sequences and series, it may be necessary to find a specific term or the sum of a series. To aid in these calculations, here are some useful formulas:

#### Formulas for Sequences

There is a formula for both arithmetic and geometric sequences. The formula for finding a term in an arithmetic sequence is:

an = a1 + (n-1)d

Where 'an' is the nth term, 'a1' is the first term, 'n' is the number of the term, and 'd' is the common difference.

For example, to find the 6th term of the sequence 5, 12, 19, 26, 33, ..., we can use this formula as follows:

Example:

Find the 6th term of this sequence:

5, 12, 19, 26, 33, ...

Solution:

Substituting the given values into the formula, we get:

a6 = 5 + (6-1)7 = 5 + 35 = 40

Therefore, the 6th term of this sequence is 40.

The formula for finding a term in a geometric sequence is:

an = a1rn-1

Where 'an' is the nth term, 'a1' is the first term, 'n' is the number of the term, and 'r' is the common ratio.

For example, to find the 6th term of the sequence 4, 8, 16, 32, 64, ..., we can use this formula as follows:

Example:

Find the 6th term of this sequence:

4, 8, 16, 32, 64, ...

Solution:

Substituting the given values into the formula, we get:

a6 = 4 x 26-1 = 4 x 25 = 128

Therefore, the 6th term of this sequence is 128.

#### Formulas for Series

The formula for finding the sum of the first 'n' terms in an arithmetic series is:

Sn = n/2[2a + (n-1)d]

Where 'Sn' is the sum of the first 'n' terms, 'n' is the number of terms, 'a' is the first term, and 'd' is the common difference.

For example, to find the sum of the first 35 terms in the series 2, 8, 14, 20, 26, 32, ..., we can use this formula as follows:

Example:

Find the sum of the first 35 terms in this series:

2, 8, 14, 20, 26, 32, ...

Solution:

Substituting the given values into the formula, we get:

S35 = 35/2[2(2) + (35-1)6] = 35/2[4 + 204] = 35(104) = 3640

Therefore, the sum of the first 35 terms in this series is 3640.

When working with geometric series, there are two different formulas depending on the value of the common ratio, 'r'.

## Understanding Sequences and Series

Sequences and series are important concepts in mathematics used to represent patterns and relationships between numbers. A sequence is a set of numbers that follow a specific rule, and a series is the sum of a sequence.

### Types of Sequences

There are two main types of sequences: arithmetic and geometric. An arithmetic sequence is characterized by adding a fixed number to the previous term to get the next term. For example, starting with 2 and adding 3 each time, we get: 2, 5, 8, 11, 14... On the other hand, a geometric sequence involves multiplying the previous term by a fixed number to get the next term. For instance, starting with 2 and multiplying by 3 each time, we get: 2, 6, 18, 54, 162...

### The Concept of Series

A series is the sum of a sequence, and it can be finite or infinite depending on the sequence. Series are represented by the Greek letter sigma (∑) followed by the terms of the sequence. For example, the series of the arithmetic sequence mentioned earlier would be written as: ∑(2 + 3n) = 2 + 5 + 8 + 11 + 14 + ...

## Simplifying Series with Formulas

To calculate the sum of a sequence, we use specific formulas depending on the type of sequence. For example, to find the sum of an arithmetic series, we use the formula Sn = n/2 (2a + (n-1)d), where Sn is the sum of the first n terms, a is the first term, and d is the common difference. Similarly, for a geometric series, we use the formula Sn = a(1 - rn)/1-r, where a is the first term and r is the common ratio.

### Applying Sequences and Series

Sequences and series have many real-life applications, also known as modeling. A common example is in financial situations, where we can use geometric series to calculate savings or investments over time. For instance, if Dave deposits \$10 in the first month, \$20 in the second month, \$40 in the third month, and so on for a year, what will be his total savings?

## Exploring Sigma Notation

The Greek letter sigma is used to represent the sum in mathematical notation. To use sigma notation, write the limits above and below sigma to show the terms being added. For example:

S = ∑ an = a1 + a2 + a3 + ... + an

This notation indicates that we will find the sum of the sequence by substituting values from 1 to 'n' into the given equation.

## Applications of Sequences and Series

Sequences and series have various real-life applications that involve modeling. For instance, arithmetic sequences can be used to represent the growth of a population, where each term represents the number of individuals added to the previous population. Geometric sequences, on the other hand, can be used to represent exponential growth or decay situations, such as compound interest or radioactive decay.