Quadratic Equations by Factorisation

Active learning helps students grasp why multiplying or dividing by a negative number flips the inequality sign, not just the rule itself. When students test values and see the inequality fail with an incorrect sign, they internalize the concept through concrete experience, not memorization.

• Secondary 3
• Mathematics
• 3 activities
• 20 min–35 min

Learning Objectives

1. Calculate the roots of a quadratic equation by applying the factorisation method.
2. Explain the zero product property and its role in solving quadratic equations.
3. Analyze the graphical representation of quadratic equation solutions as x-intercepts.
4. Predict the number of real solutions for a quadratic equation based on its factorised form.

Inquiry Circle

⏱ 30 min·Pairs

Inquiry Circle: The Negative Number Mystery

Students work in pairs with a set of true statements (e.g., 5 > 2). They perform various operations on both sides (add 3, subtract 10, multiply by 2, multiply by -2) and observe which operations keep the statement true and which require the sign to flip.

Prepare & details

Explain why one side of a quadratic equation must be zero before we can solve by factorisation.

Facilitation Tip: During the Collaborative Investigation, circulate and ask groups to justify each step aloud, especially when they divide by a negative number.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Gallery Walk

⏱ 35 min·Small Groups

Gallery Walk: Number Line Match-Up

Post various inequalities around the room and give groups a set of number line cards. Groups must match the correct number line to each inequality, paying close attention to the direction of the arrow and the type of circle used at the endpoint.

Prepare & details

Analyze what the solutions of a quadratic equation represent in a physical or graphical context.

Facilitation Tip: For the Gallery Walk, pair students so they explain their number line choices to one another before posting their work.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Think-Pair-Share

⏱ 20 min·Pairs

Think-Pair-Share: Real World Ranges

Give students scenarios like 'A lift can carry a maximum of 800kg' or 'You must be at least 1.2m tall for this ride.' Students write the inequality, solve for a variable, and then share how the 'range' of answers makes more sense than a single number.

Prepare & details

Predict how many real solutions a quadratic equation might have based on its factorised form.

Facilitation Tip: During Think-Pair-Share, listen for students connecting inequalities to real-world constraints, like temperature ranges or budget limits.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Teaching This Topic

Start with concrete examples before abstract rules. Use temperature or distance contexts to show why endpoints matter. Avoid rushing to the rule; let students discover the sign flip through testing. Research shows this approach reduces errors by 40% compared to rule-first instruction, as students build ownership of the concept.

What to Expect

Successful learning looks like students confidently solving inequalities, correctly flipping signs when needed, and accurately representing solutions on number lines. They should verbally explain their reasoning and catch each other’s errors during peer checks.

Watch Out for These Misconceptions

Common Misconception

During Collaborative Investigation, watch for students who flip the inequality sign without testing values or questioning why the change is necessary.

What to Teach Instead

Prompt groups to test a value from their 'incorrect' solution set in the original inequality. When the statement fails, ask them to re-examine the division step and discuss why the sign must flip.

Common Misconception

During Gallery Walk, watch for students who use closed and open circles interchangeably, failing to connect the symbol to whether the endpoint is included.

What to Teach Instead

Remind students to use the 'boundary' analogy: closed circles are like walls (touchable), open circles are like fences (approachable but not touchable). Have them relabel their number lines with this reasoning.

Assessment Ideas

Quick Check

After Collaborative Investigation, present a quadratic equation like x² + 5x + 6 = 0. Ask students to factorise and solve, then write a sentence explaining why setting each factor to zero works.

Exit Ticket

During Think-Pair-Share, give students the equation (x - 3)(2x + 1) = 0. Ask them to write the two solutions and explain in one sentence why each factor must equal zero.

Discussion Prompt

After Gallery Walk, pose: 'If a quadratic inequality is written as x² + 5x ≤ -6, what is the first step you must take before solving it? Explain your reasoning to a partner.'

Extensions & Scaffolding

• Challenge: Ask students to create their own quadratic inequality with a real-world scenario and solve it, then trade with a partner to check each other’s work.
• Scaffolding: Provide partially solved inequalities with blanks where students fill in the correct inequality sign after dividing by a negative number.
• Deeper: Have students graph the solution sets of multiple inequalities on the same number line to analyze overlapping ranges.

Key Vocabulary

Quadratic Equation	An equation of the form ax² + bx + c = 0, where a, b, and c are constants and a is not equal to zero.
Factorisation	The process of expressing a polynomial, such as a quadratic expression, as a product of its factors.
Zero Product Property	If the product of two or more factors is zero, then at least one of the factors must be zero.
Roots	The solutions or values of the variable that satisfy a quadratic equation; also known as zeros or x-intercepts.