How to Find Arithmetic Sequence – A Step-by-Step Guide

Feature Image What Is the Difference Between Arithmetic and Geometric Sequences Understanding with Examples

To find an arithmetic sequence, I first identify the common difference between consecutive terms. This difference is constant and is denoted by (d). For any given arithmetic sequence, the (n)th term can be calculated using the formula $a_n = a_1 + (n-1)d$, where $a_1$ is the first term and (n) is the term number.

Finding the sum of the first (n) terms involves a different formula, namely $S_n = \frac{n}{2}(2a_1 + (n-1)d$, which is derived from the sequence’s properties.

I always make sure to have the sequence’s initial term and the common difference on hand, as these are critical for plugging into the formulas. The process itself is straightforward once these values are known.

I find it satisfying how a simple pattern of adding or subtracting the same number can reveal so much about a series of numbers. Stick around, and I’ll walk you through understanding each component in more detail.

Steps Involved in Finding Arithmetic Sequences

When I look at a sequence, identifying whether it’s an arithmetic sequence is my first step. An arithmetic sequence is a sequence of numbers where each term after the first is found by adding a consistent number, known as the common difference, to the previous term.

This common difference can be positive, negative, or zero, leading to increasing, decreasing, or constant sequences, respectively.

To identify and work with an arithmetic sequence, I follow these steps:

  1. Identify the Common Difference
    The common difference ((d)) is found by subtracting any term from the subsequent term. For any two consecutive terms $a_n$ and $a_{n+1}$, it’s calculated as: $d = a_{n+1} – a_n $
    This difference must remain constant throughout the sequence.

  2. Determine the First Term
    The first term $a_1$ is often provided, but if not, I back-calculate it using the common difference and a term whose position is known.

  3. Derive the General Formula
    Once (d) and $a_1$ are known, I use them to compose the general formula for the (n)th term of the sequence: $a_n = a_1 + (n-1)d$

  4. Validate the Sequence
    To confirm if a sequence is arithmetic, I check if the pattern of adding the common difference to each term to get to the next term is consistent throughout.

  5. Use the Pattern
    With the general formula, any term in the sequence can be found. For example, the 12th term $a_{12}$ is found by plugging (n = 12) into the formula.

Remember, arithmetic sequences are a foundational concept in algebra, and understanding them can be very useful in grasping related math lessons and tutorials. Unlike geometric sequences or Fibonacci sequences, where terms are multiplied or are a sum of previous terms, arithmetic sequences thrive on the simplicity of addition or subtraction.

For sequences involving fractions or negative numbers, the same steps apply but with additional attention to maintaining the correct sign and appropriately adding or subtracting fractions.

Calculating Terms in an Arithmetic Sequence

In an arithmetic sequence, each term is equal to the previous term plus a constant difference. To find any term, I use the arithmetic sequence formula:

$a_n = a_1 + (n-1)d$

Here, $a_n$ is the nth term I’m looking to find, $a_1$ is the first term, and ( d ) is the common difference. The term ( n ) stands for the term’s position in the sequence.

For instance, if I have an arithmetic sequence where the first term $a_1$ is 5 and the common difference ( d ) is 3, and I want to calculate the 4th term $a_4$, I apply the formula:

$a_4 = 5 + (4-1) \times 3$

$ a_4 = 5 + 9$

$a_4 = 14$

I can also represent sequences with an explicit formula:

$a_n = a_1 + (n-1)d$

Or a recursive formula, which expresses each term based on the previous one:

$a_{n} = a_{n-1} + d$

Here’s a simple table to illustrate the calculation of terms in the sequence with $a_1 = 5$ and ( d = 3 ):

Term (n)CalculationResult
25 + 38
38 + 311
411 + 314
514 + 317

Each entry under “Calculation” follows the pattern of adding the common difference to the previous term to get the next term. This way, I can build the entire sequence term by term, or calculate any specific term using the initial value and the common difference.

Summing Terms in Arithmetic Series

When I work with an arithmetic series, the goal is to find the sum of all terms within the sequence. An arithmetic series is a sequence of numbers in which each term after the first is found by adding a constant, known as the common difference, to the previous term.

For example, if I start with the term 3 and the common difference is 2, my arithmetic series would be: 3, 5, 7, 9, and so on. To find the sum of the first ‘n’ terms of this series, I use the arithmetic series formula:

$$ S_n = \frac{n}{2} \times (a_1 + a_n) $$

Here, ‘$S_n$’ is the sum of the first ‘n’ terms, ‘$a_1$’ is the first term, and ‘$a_n$’ is the nth term. The nth term can also be found using the formula:

$$ a_n = a_1 + (n – 1) \times d $$

Where ‘d’ is the common difference. Let’s consider we want to find the sum of the first 5 terms from my example. The first term, ‘$a_1$’, is 3, and the fifth term, ‘$a_5$’, is 11 ($since 3 + 4 \times 2 = 11$). Plugging these values into the formula gives us:

$$ S_5 = \frac{5}{2} \times (3 + 11) = \frac{5}{2} \times 14 = 35 $$

Thus, the sum of the first 5 terms is 35. The table below illustrates the sum of an arithmetic series depending on the number of terms ‘n’:

nFirst Term (a_1)Common Difference (d)nth Term $a_n$Sum $S_n$

Finding the sum of an arithmetic series is straightforward with the right formula. Remember to identify the first term, common difference, and the nth term for accurate solutions.


Through this article, I’ve guided you through the steps to identify and work with arithmetic progressions. Recognizing the common difference is key, whether it’s positive, indicating an increasing sequence, or negative, for a decreasing sequence.

Unlike geometric sequences where terms are multiplied, or the Fibonacci sequence that adds the two previous terms, arithmetic sequences have a straightforward pattern: each term increments by a fixed amount.

Finding the n-th term of an arithmetic sequence involves the formula $a_n = a_1 + (n-1)d$, where $a_1$ is the first term and $d$ is the common difference. Similarly, the sum of an arithmetic series can be found using $S_n = \frac{n}{2}(a_1 + a_n)$ or $S_n = \frac{n}{2}(2a_1 + (n-1)d)$.

I aimed to make algebra accessible, especially if dealing with fractions or negative numbers within these sequences.

To further your understanding, related math lessons, and tutorials are available, often spotlighting the domain and function of sequences in algebra. These resources cater to both beginners and those looking to polish their skills, covering a variety of topics beyond what we’ve explored together.

Remember, practice makes perfect. Applying these formulas to different problems will bolster your arithmetic sequence skills. Stay curious, and don’t hesitate to dive into more complex sequences as you grow more confident with these basics.