Which Formula Can Be Used to Describe the Sequence – A Complete Guide

As we embark on this exploration, we’ll unravel which formula can be used to describe the sequence, illuminating the hidden symphony that plays within the realm of numbers.

Which Formula Can Be Used to Describe the Sequence?

To describe a sequence, we can use the general term formula \(a_n\)​. For arithmetic sequences, it’s \(a_n\) ​= \(a_1\) ​+ (n − 1) d and for geometric sequences, it’s \(a_n\) ​= \(a_1\) ​× r (n − 1), where \(a_1\)​ is the first term, $d$ is the common difference, and $r$ is the common ratio.

At the heart of every sequence lies a formula—a key that unlocks the pattern and aids us in determining any term of the sequence, no matter how far it may be in the progression. So, how do we discern which formula encapsulates a given sequence? Let’s dive into this intricate world of numbers and patterns.

Mathematics, a subject known for its precision, also embraces the beauty of patterns and sequences. From the simple arithmetic sequences encountered in elementary school to the more intricate sequences studied in advanced mathematics, there’s an inherent beauty in observing the structured progression of numbers.

Understanding Sequences

A sequence can be visualized as a list of numbers arranged in a specific order. Each number in this list is called a ‘term’. When we talk about the “nth term” of a sequence, we are referring to the term that’s in the ‘n’ position in the sequence. The progression or pattern observed in the sequence is what we aim to capture using formulas.

Explicit vs. Recursive Formulas

Broadly, formulas used to describe sequences fall under two categories: explicit and recursive.

Explicit Formula

This type of formula provides a direct relationship between the n-th term and the number $n$. It allows you to find the nth term without having to know any of the preceding terms. For example, consider the sequence of even numbers: 2$,4,6,8,\dots$. This can be described by the explicit formula \(a_n\) ​= 2 * n.

Recursive Formula

Unlike explicit formulas, recursive formulas define the terms of a sequence in relation to one or more of its preceding terms. A classic example is the Fibonacci sequence, where each term is derived from the sum of the two preceding terms: \(a_n\) = \(a_{n-1}\)​ + \(a_{n-2}\)​. To use a recursive formula, you must know one or more initial terms.

Discovering the Formula

Unearthing the formula that encapsulates a sequence is akin to a mathematical detective mission. Here are some steps to guide the way:

Observation

Start by observing the first few terms. Do you notice any consistent increase or decrease? This might hint towards an arithmetic or geometric sequence.

Differences and Ratios

Calculate the differences between consecutive terms for potential arithmetic patterns, or ratios for geometric sequences. If the differences or ratios are constant, you’re on the right track.

Utilize Known Formulas

Familiarize yourself with common sequences and their formulas. For instance, arithmetic sequences follow $\backslash (a\_\{n\}\backslash )$ ​= \(a_{1}\)​+ (n−1) d where $d$ is the common difference. Geometric sequences, on the other hand, adhere to $\backslash (a\_\{n\}\backslash )$ ​= \(a_{1}\) ​× r (n − 1) where $r$ is the common ratio.

Experimentation

Sometimes, sequences don’t fall neatly into known categories. In such cases, playing with the numbers, graphing them, or using computational tools can provide insights.

Exercise

Example 1

Sequence: 2, 5, 8, 11, 14…

Solution

The difference between consecutive terms is constant and equal to 3.

Formula:  

$\backslash (a\_\{n\}\backslash )$=  \(a_{1}\) ​+ (n−1) d

Here,  $\backslash (a\_\{1\}\backslash )$​ = 2 and $d=3$. Thus:

 \(a_{n}\) = 2 + 3(n−1)

= $3n-1$

Example 2

Sequence: 3, 6, 12, 24, 48…

Solution

The ratio between consecutive terms is constant and equal to 2.

Formula:

 \(a_{n}\) ​=  \(a_{1}\) ​× \(r^{n-1}\)

Here,  $\backslash (a\_\{1\}\backslash )$​= 3 and $r=2$. Thus:

 \(a_{n}\) ​= 3 × 2(n−1)

Example 3

Sequence: 1, 4, 9, 16, 25…

Solution

Each term is the square of a whole number.

Formula:

 \(a_{n}\) = \(n^{2}\)

Example 4

Sequence: 1, 1, 2, 3, 5, 8…

Solution

Each term is the sum of the two preceding terms.

Recursive Formula:

 \(a_{n}\) ​=  \(a_{n-1}\) ​+  \(a_{n-2}\)​

With initial conditions:

 \(a_{1}\) ​= 1, and  \(a_{2}\) ​= 1

Applications and Beyond

Understanding sequences and their underlying formulas is not just a theoretical exercise. It has significant practical implications. Financial analysts use geometric sequences to model investments and interest over time.

Biologists turn to sequences like the Fibonacci series when studying population growth or patterns in nature. Even in computer algorithms, sequence formulas play a pivotal role in optimizing tasks and operations.

Moreover, some sequences, such as the prime numbers, still harbor mysteries that mathematicians have been trying to unravel for centuries. The patterns (or seeming lack thereof) observed in these sequences and the hunt for their underlying formulas have been the subject of numerous research papers, discussions, and debates.