Sequences

Sequences follow patterns that can be described using rules such as the nth term. Recognising and forming these rules is an important algebra skill.

Overview

A sequence is a list of numbers that follow a rule or pattern.

Each number in the list is called a term.

\( 3,\ 7,\ 11,\ 15,\ 19,\dots \)

In GCSE Maths, you need to continue sequences, spot the pattern, and sometimes find a formula for the nth term.

What you should understand after this topic

  • Understand what a sequence is
  • Spot common differences
  • Continue a sequence
  • Find the nth term of a linear sequence
  • Recognise when a sequence is not linear

Key Definitions

Sequence

A list of numbers that follow a rule or pattern.

Term

Each individual number in a sequence.

First Term

The number that appears first in the sequence.

Common Difference

The amount added or subtracted each time in an arithmetic sequence.

Arithmetic Sequence

A sequence where the difference between consecutive terms is always the same.

nth Term

A formula that gives any term in the sequence.

Linear Sequence

A sequence with a constant first difference.

Non-linear Sequence

A sequence where the difference does not stay the same.

Key Rules

Look at the differences

Subtract consecutive terms to spot the pattern.

Same difference?

If the difference stays the same, the sequence is arithmetic.

nth term form

For linear sequences, nth term is usually in the form \( an + b \).

Check your rule

Test your nth term with the first few term numbers.

Quick Pattern Check

Increasing sequence

\( 4,\ 7,\ 10,\ 13,\dots \)

Decreasing sequence

\( 20,\ 16,\ 12,\ 8,\dots \)

Square numbers

\( 1,\ 4,\ 9,\ 16,\dots \)

Triangular style pattern

\( 1,\ 3,\ 6,\ 10,\dots \)

How to Solve

What is a sequence?

A sequence is an ordered list of numbers. Each term follows a rule or pattern.

\( 2,\ 5,\ 8,\ 11,\ 14,\dots \)
This sequence increases by 3 each time.
Key idea: The rule links one term to the next.

Step 1: Find the difference

Check how much each term changes by.

\( 6,\ 10,\ 14,\ 18,\dots \)
\(10 - 6 = 4\)
\(14 - 10 = 4\)
\(18 - 14 = 4\)
Same difference → arithmetic (linear) sequence.

Step 2: Continue the sequence

Use the pattern to find the next terms.

\( 9,\ 13,\ 17,\ 21,\dots \)
Add 4 each time.
Next terms: \(25,\ 29,\ 33\).

Step 3: Find the nth term (linear sequences)

The nth term gives a formula for any position in the sequence.

\( an + b \)
\(a\) = common difference.
\(b\) adjusts the sequence to match the first term.

Method: Finding the nth term

  1. Find the common difference (this is \(a\)).
  2. Write the sequence \(a,\ 2a,\ 3a,\dots\).
  3. Compare with the actual sequence.
  4. Adjust by adding or subtracting a constant.

Worked example 1

\( 4,\ 7,\ 10,\ 13,\dots \)

Worked example 2

\( 12,\ 9,\ 6,\ 3,\dots \)

Non-linear sequences

If the difference is not constant, the sequence is not linear.

\( 1,\ 4,\ 9,\ 16,\ 25,\dots \)
Differences: \(3,\ 5,\ 7,\ 9\).
This is a quadratic (square number) sequence.
Sequence of square numbers plotted showing increasing gaps between terms

Exam thinking

Always check the difference first.
If constant → linear sequence.
If not constant → look for a pattern (e.g. squares).
Exam tip: Show how you built the nth term, not just the answer.

Exam method summary

See quadratic graphs for how square patterns appear visually.
  1. Find the difference.
  2. Decide if the sequence is linear.
  3. Use \(an\) as a starting point.
  4. Adjust to match the sequence.
  5. Check your rule works for multiple terms.

Example Questions

Edexcel

Exam-style questions inspired by Edexcel GCSE Mathematics.

Edexcel

Write down the next two terms in the sequence: 3, 7, 11, 15, ...

Edexcel

Find the next term in the sequence: 2, 6, 18, 54, ...

Edexcel

Find the nth term of the sequence: 4, 7, 10, 13, ...

Edexcel

Find the 20^{\text{th}} term of the sequence with nth term \( 3n + 2 \).

Edexcel

Is 95 a term in the sequence defined by \( 5n - 5 \)? Show your working.

AQA

Exam-style questions based on the AQA GCSE Mathematics specification, focusing on algebraic reasoning and forming rules.

AQA

Find the nth term of the sequence: 5, 9, 13, 17, ...

AQA

Find the nth term of the sequence: 2, 5, 10, 17, 26, ...

AQA

The nth term of a sequence is \( 4n - 1 \). Write down the first four terms.

AQA

The nth term of a sequence is \( n^2 + 2 \). Find the 10th term.

AQA

Explain why the sequence defined by \( 2n + 3 \) will never contain the number 50.

OCR

Exam-style questions aligned with OCR GCSE Mathematics, emphasising reasoning, patterns, and real-life applications.

OCR

Here is a sequence: 1, 4, 9, 16, 25, ... Write down the next term and state the rule.

OCR

Find the nth term of the sequence: 3, 8, 15, 24, 35, ...

OCR

A pattern is made using matchsticks. The number of matchsticks in each stage forms the sequence 4, 7, 10, 13, ... Find a formula for the nth term.

OCR

Given that the nth term of a sequence is \( 2n^2 - n \), find the 5^{\text{th}} term.

OCR

Determine whether 132 is a term in the sequence defined by \( n^2 + n \). Show your reasoning.

Exam Checklist

Step 1

Look carefully at how the sequence changes from one term to the next.

Step 2

Find the common difference if the sequence is linear.

Step 3

Use the difference to build the nth term.

Step 4

Check your nth term with term 1, term 2 and term 3.

Most common exam mistakes

Wrong difference

A small subtraction error can make the whole nth term wrong.

Wrong adjustment

Students often choose the right multiple of \(n\) but add or subtract the wrong number.

No checking

Always test the nth term on the first few terms.

Misreading pattern

Some sequences are non-linear, so a linear rule will not work.

Common Mistakes

These are common mistakes students make when working with sequences in GCSE Maths.

Using the wrong common difference

Incorrect

A student calculates the step between terms incorrectly.

Correct

Find the difference between consecutive terms carefully. Check more than one pair to confirm the pattern.

Starting term numbers at 0

Incorrect

A student substitutes \(n = 0\) for the first term.

Correct

In GCSE sequences, the first term usually corresponds to \(n = 1\), not 0.

Wrong adjustment in the nth term

Incorrect

A student finds the correct multiple of \(n\) but adds or subtracts the wrong number.

Correct

After finding the multiple (from the common difference), compare with the first term to determine the correct adjustment.

Assuming all sequences are linear

Incorrect

A student applies a linear formula to a non-linear sequence.

Correct

Check whether the differences are constant. If not, the sequence may be quadratic or follow a different pattern.

Not checking the rule

Incorrect

A student writes an nth term without verifying it.

Correct

Substitute values of \(n\) into your formula to check it produces the given terms.

Try It Yourself

Practise identifying and extending number sequences.

Questions coming soon
Foundation

Foundation Practice

Find patterns and continue sequences.

Question 1

What is the next number in the sequence: 3, 6, 9, 12, ... ?

Games

Practise this topic with interactive games.

Games coming soon.

Frequently Asked Questions

What is a sequence?

A list of numbers that follow a pattern.

What is the nth term?

A formula that gives any term in the sequence.

How do I find the nth term?

Look at how the sequence changes and relate it to position.