Factorisation by Common Factors

Active learning helps students grasp factorisation by common factors because moving, sorting, and correcting errors turns abstract symbols into tangible operations. When students physically manipulate terms, they see how grouping works rather than memorising rules, which builds deeper understanding of equivalence and simplification.

• Secondary 3
• Mathematics
• 4 activities
• 25 min–40 min

Learning Objectives

1. Identify the greatest common numerical and algebraic factors in given expressions.
2. Calculate the greatest common factor (GCF) for sets of terms within an algebraic expression.
3. Factor algebraic expressions by extracting the GCF, including binomial common factors.
4. Compare the expanded form of an expression with its factored form to demonstrate equivalence.
5. Construct an argument explaining why factorisation is the inverse operation of expansion.

Problem-Based Learning

⏱ 30 min·Pairs

Card Sort: Factor Matches

Prepare cards with unfactored expressions on one set and factored forms on another. Pairs sort and match them, then justify pairings on mini-whiteboards. Groups share one challenging match with the class for verification by expansion.

Prepare & details

Explain how finding the greatest common factor simplifies an expression.

Facilitation Tip: During Card Sort: Factor Matches, have students verbalise their reasoning for each pair before gluing it down, forcing them to justify their choices.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 40 min·Small Groups

Relay Challenge: Common Factors

Divide class into small groups and line them up. Provide an expression; first student factors out one common part and passes to the next, who continues until complete. Groups race, then check by expanding.

Prepare & details

Differentiate between common numerical factors and common algebraic factors.

Facilitation Tip: For Relay Challenge: Common Factors, ensure each group has scratch paper to expand their factored expressions, so they can immediately verify their work.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 35 min·Small Groups

Error Hunt: Faulty Factorisations

Distribute worksheets with five incorrect factorisations. Small groups identify errors, correct them, and explain why. Present findings to class, voting on most common pitfalls.

Prepare & details

Construct an argument for why factorisation is the reverse of expansion.

Facilitation Tip: In Error Hunt: Faulty Factorisations, ask students to write corrections directly on the sheet, then discuss their answers with another pair to compare reasoning.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 25 min·Pairs

Binomial Builder: Create and Factor

Individuals generate expressions with binomial common factors, then swap with partners to factor. Partners expand to verify. Discuss patterns in a whole-class debrief.

Prepare & details

Explain how finding the greatest common factor simplifies an expression.

Facilitation Tip: For Binomial Builder: Create and Factor, circulate and ask groups to explain how the binomial factor connects to each term before they write the final expression.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Teaching This Topic

Teach this topic by starting with numerical examples to build confidence, then gradually introduce algebraic terms. Avoid rushing to binomial factors; ensure students are solid on single-variable cases first. Research suggests that students learn factorisation best when they alternate between expanding and factoring, as this strengthens their understanding of equivalence. Use consistent language like 'take out' or 'extract' to avoid confusion with division terminology.

What to Expect

Successful learning looks like students confidently identifying the greatest common factor in both numerical and algebraic expressions and explaining why factorisation maintains equivalence. Students should also articulate how grouped terms relate to the original expression and use precise language to describe the process.

Watch Out for These Misconceptions

Common Misconception

During Card Sort: Factor Matches, watch for students who only compare numerical coefficients and ignore matching variable terms.

What to Teach Instead

Have students highlight variable parts in different colors and physically group terms with matching bases and powers during the sort.

Common Misconception

During Relay Challenge: Common Factors, watch for students who believe factorisation changes the value of the expression.

What to Teach Instead

Require groups to expand their factored expressions on a whiteboard and visually compare with the original to confirm equivalence before moving to the next expression.

Common Misconception

During Error Hunt: Faulty Factorisations, watch for students who miss binomial factors because they do not see them as single units.

What to Teach Instead

Ask students to circle each binomial factor before attempting to correct the expression, reinforcing that it functions as one unit even when split across terms.

Assessment Ideas

Quick Check

After Card Sort: Factor Matches, collect students' sorted pairs and assess whether they correctly identified both numerical and algebraic common factors in each pair.

Exit Ticket

After Relay Challenge: Common Factors, ask students to complete an exit ticket with two expressions to factor completely and a brief explanation of how they identified the common factor.

Discussion Prompt

During Binomial Builder: Create and Factor, listen for students to explain how the binomial factor connects to each term in their expressions, noting whether they describe it as a shared unit.

Extensions & Scaffolding

• Challenge: Provide expressions with three terms, such as 6xy + 9xz + 12xw, and ask students to factor out the GCF and explain why it works.
• Scaffolding: Give students a partially completed factorisation, such as 4a( ) + 8b( ), and ask them to fill in the blank with a binomial.
• Deeper: Ask students to create their own expressions with a common binomial factor, then swap with a partner to factor and verify each other's work.

Key Vocabulary

Factor	A number or algebraic expression that divides another number or expression evenly. For example, 3 and x are factors of 6x.
Common Factor	A factor that two or more numbers or expressions share. For example, 2 is a common factor of 4 and 6.
Greatest Common Factor (GCF)	The largest factor that two or more numbers or expressions have in common. For example, the GCF of 12x and 18x^2 is 6x.
Binomial Factor	A factor that consists of two terms, such as (x + 2). This can be a common factor in more complex expressions.
Algebraic Expression	A mathematical phrase that can contain numbers, variables, and operation symbols. For example, 3x + 6 is an algebraic expression.