Arithmetic Sequences

Learn to identify, analyze, and work with arithmetic sequences where terms increase or decrease by a constant difference.

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Definition

An arithmetic sequence is a sequence of numbers where the difference between consecutive terms is always the same. This constant difference is called the common difference and is denoted by

d

For example:

3, 7, 11, 15, 19, \ldots

Here, each term is 4 more than the previous term, so

d = 4

The Explicit Formula

To find any term in an arithmetic sequence, use the explicit formula:

a_n = a_1 + (n - 1) \cdot d

Where:

$a_n$ = the $n$ th term (the term we want to find)
$a_1$ = the first term
$n$ = the position of the term
$d$ = the common difference

Finding the Common Difference

To find

d

, subtract any term from the term that follows it:

d = a_{n+1} - a_n

Try it now

Which sequence is arithmetic?

Worked Examples

Find the common difference for the sequence: $8, 5, 2, -1, -4, \ldots$

Choose two consecutive terms

Let's use $a_1 = 8$ and $a_2 = 5$ → First two terms identified

Subtract the first from the second

$d = a_2 - a_1 = 5 - 8 = -3$ → $d = -3$

Verify with another pair

$2 - 5 = -3$ and $-1 - 2 = -3$ → Confirmed: $d = -3$

Answer: The common difference is $d = -3$ . This is a decreasing arithmetic sequence.

Common Mistakes

Using $n$ instead of $(n-1)$ in the formula

Why it's wrong: The formula $a_n = a_1 + nd$ would give you the $(n+1)$ th term, not the $n$ th term. When $n=1$ , we should get $a_1$ , which requires $(n-1) = 0$ .

Correct: Always use $a_n = a_1 + (n-1) \cdot d$ . Check: when $n=1$ , $a_1 = a_1 + 0 \cdot d = a_1$ .

Subtracting in the wrong order when finding $d$

Why it's wrong: Students sometimes calculate $a_n - a_{n+1}$ instead of $a_{n+1} - a_n$ , which gives the wrong sign for $d$ .

Correct: Always subtract the earlier term from the later term: $d = a_{n+1} - a_n$

Forgetting that $d$ can be negative

Why it's wrong: Arithmetic sequences can decrease! A sequence like $20, 17, 14, 11, \ldots$ has $d = -3$ .

Correct: Check if the sequence is increasing ( $d > 0$ ) or decreasing ( $d < 0$ ) before writing the formula.

Confusing the term number with the term value

Why it's wrong: In $a_5 = 23$ , the subscript 5 is the position (5th term), and 23 is the value.

Correct: $a_n$ means the term at position $n$ . The subscript is always the position.

Watch a video explanation

The same topic explained by another teacher, if a video helps you more.

Arithmetic Sequences Explained: nth Term and Partial Sums
by Understand The Math
Watch on YouTube

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Practice Problems

17 problems

Problem 1 of 17

Easy

Which sequence is arithmetic?

Why It Matters

Arithmetic sequences appear everywhere in daily life and are fundamental to understanding patterns:

Savings Plans: If you save 50 dollars each month, your total savings form an arithmetic sequence: 50, 100, 150, 200, ...
Seating Arrangements: Theater rows often increase by a fixed number of seats: 20, 24, 28, 32, ...
Depreciation: A car losing 2000 dollars in value each year follows an arithmetic pattern
Stair Construction: Each step rises by the same height, forming an arithmetic sequence of elevations

Understanding arithmetic sequences leads to the study of arithmetic series (sums) and prepares you for more complex patterns like geometric sequences.

Real World Applications

Savings and Financial Planning

Regular savings deposits create arithmetic sequences. Understanding these patterns helps with financial planning.

Example:

If you save 75 euros per month starting from 0, after $n$ months you have $75n$ euros. If you start with 200 euros, your balance forms the sequence $200, 275, 350, 425, \ldots$

1Try It Yourself

You open a savings account with 500 euros and deposit 125 euros every month.

How much will you have after 2 years (24 months)?

Step 1: Write the mathematical expression

Use $a_n = a_1 + (n-1) \cdot d$ with $a_1 = 500$ and $d = 125$

Construction and Architecture

Builders use arithmetic sequences when planning staircases, seating, and structural patterns.

Example:

A staircase has a total rise of 3 meters over 15 steps. Each step rises by $\frac{3}{15} = 0.2$ meters. The heights form the sequence $0.2, 0.4, 0.6, \ldots, 3.0$ meters.

2Try It Yourself

A pyramid of cans has 1 can on top, 3 in the second row, 5 in the third row, and so on.

How many cans are in the 10th row?

Step 1: Write the mathematical expression

This is an arithmetic sequence with $a_1 = 1$ and $d = 2$

Sports and Fitness

Training programs often increase intensity following arithmetic patterns.

Example:

A runner starts with 2 km and adds 0.5 km each week. After $n$ weeks, they run $2 + 0.5(n-1)$ km. Week 10: $2 + 0.5 \times 9 = 6.5$ km.

3Try It Yourself

A gym membership costs 30 euros to join plus 15 euros per month.

What is the total cost after 12 months?

Step 1: Write the mathematical expression

First month total is 45 euros, then add 15 each month

Key Takeaways

1An arithmetic sequence has a constant difference $d$ between consecutive terms
2The explicit formula is $a_n = a_1 + (n-1) \cdot d$
3Find $d$ by subtracting any term from the next: $d = a_{n+1} - a_n$
4The common difference $d$ can be positive (increasing sequence) or negative (decreasing sequence)
5To find a specific term, substitute the position $n$ into the explicit formula

Frequently Asked Questions

A sequence is a list of numbers following a pattern (e.g.,

2, 5, 8, 11, \ldots

). A series is the sum of the terms in a sequence (e.g.,

2 + 5 + 8 + 11 + \ldots

). We study arithmetic series after mastering sequences.

Glossary

Arithmetic sequence: A sequence where each term differs from the previous term by a constant amount called the common difference
Common difference: The constant value $d$ added to each term to get the next term in an arithmetic sequence
Explicit formula: A formula that directly calculates any term: $a_n = a_1 + (n-1) \cdot d$
Term: Each number in a sequence; $a_n$ represents the term at position $n$
First term: The starting value of the sequence, denoted $a_1$
nth term: The term at position $n$ in the sequence, denoted $a_n$

Formula Card

Explicit Formula

a_n = a_1 + (n-1) \cdot d

Find the $n$th term directly

Common Difference

d = a_{n+1} - a_n

Difference between consecutive terms

Alternative Form

a_n = a_m + (n-m) \cdot d

Find $a_n$ using any known term $a_m$

Arithmetic Sequences

Definition

The Explicit Formula

Finding the Common Difference

Worked Examples

Common Mistakes

Watch a video explanation

Interactive Visual

Interactive Sandbox

Expression Calculator

History

Practice Problems

Why It Matters

Real World Applications

Savings and Financial Planning

Construction and Architecture

Sports and Fitness

Key Takeaways

Frequently Asked Questions

Glossary

Formula Card

Explicit Formula

Common Difference

Alternative Form

More in This Topic

Introduction to Sequences

Geometric Sequences