What is the next number in the sequence: 3, 6, 9, 12, ... ?
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.
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.
Step 1: Find the difference
Check how much each term changes by.
Step 2: Continue the sequence
Use the pattern to find the next terms.
Step 3: Find the nth term (linear sequences)
The nth term gives a formula for any position in the sequence.
Method: Finding the nth term
- Find the common difference (this is \(a\)).
- Write the sequence \(a,\ 2a,\ 3a,\dots\).
- Compare with the actual sequence.
- Adjust by adding or subtracting a constant.
Worked example 1
Worked example 2
Non-linear sequences
If the difference is not constant, the sequence is not linear.
Exam thinking
Exam method summary
- Find the difference.
- Decide if the sequence is linear.
- Use \(an\) as a starting point.
- Adjust to match the sequence.
- Check your rule works for multiple terms.
Example Questions
Edexcel
Exam-style questions inspired by Edexcel GCSE Mathematics.
Write down the next two terms in the sequence: 3, 7, 11, 15, ...
Find the next term in the sequence: 2, 6, 18, 54, ...
Find the nth term of the sequence: 4, 7, 10, 13, ...
Find the 20^{\text{th}} term of the sequence with nth term \( 3n + 2 \).
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.
Find the nth term of the sequence: 5, 9, 13, 17, ...
Find the nth term of the sequence: 2, 5, 10, 17, 26, ...
The nth term of a sequence is \( 4n - 1 \). Write down the first four terms.
The nth term of a sequence is \( n^2 + 2 \). Find the 10th term.
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.
Here is a sequence: 1, 4, 9, 16, 25, ... Write down the next term and state the rule.
Find the nth term of the sequence: 3, 8, 15, 24, 35, ...
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.
Given that the nth term of a sequence is \( 2n^2 - n \), find the 5^{\text{th}} term.
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
A student calculates the step between terms incorrectly.
Find the difference between consecutive terms carefully. Check more than one pair to confirm the pattern.
Starting term numbers at 0
A student substitutes \(n = 0\) for the first term.
In GCSE sequences, the first term usually corresponds to \(n = 1\), not 0.
Wrong adjustment in the nth term
A student finds the correct multiple of \(n\) but adds or subtracts the wrong number.
After finding the multiple (from the common difference), compare with the first term to determine the correct adjustment.
Assuming all sequences are linear
A student applies a linear formula to a non-linear sequence.
Check whether the differences are constant. If not, the sequence may be quadratic or follow a different pattern.
Not checking the rule
A student writes an nth term without verifying it.
Substitute values of \(n\) into your formula to check it produces the given terms.
Try It Yourself
Practise identifying and extending number sequences.
Foundation Practice
Find patterns and continue sequences.
Find the next term: 5, 10, 15, 20, ...
What is the next number: 20, 18, 16, 14, ... ?
Find the next term: 7, 14, 21, ...
Which sequence is increasing by 4 each time?
Find the next term: 50, 45, 40, ...
A student says the sequence 3, 6, 12, 24 increases by 3. What is the mistake?
Find the next term: 1, 4, 7, 10, ...
Which sequence is decreasing?
Find the next term: 100, 90, 80, ...
Higher Practice
Find rules and nth terms of sequences.
Find the nth term of: 2, 5, 8, 11, ...
Find the nth term of: 4, 7, 10, 13, ...
Find the nth term: 5, 9, 13, 17, ...
Find the nth term: 3, 6, 9, 12, ...
Find the 10th term of: 2, 5, 8, 11, ...
Find the 6th term of: 4, 7, 10, 13, ...
A student says the nth term of 2, 5, 8, 11 is \(3n + 2\). What is wrong?
Find the nth term: 7, 12, 17, 22, ...
Which nth term matches: 10, 8, 6, 4, ... ?
Find the nth term: 1, 3, 5, 7, ...
Games
Practise this topic with interactive games.
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.