← Teacher guide

Factorising

Students share algebra tiles into equal rows to factorise sx + t, learn to take out the highest common factor (numbers, then letters) and check by expanding (AC9M8A01, node 2).

Algebra Foundations · Brackets · about 45 minutes · Open the student lesson

Classroom resources

Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.

Students will be able to

Timing

Predict and Run 8 min · Investigate 15 min · Modify 12 min · Practise and Make 10 min

Before class

Do the Run board and the Make task yourself first. Students need Expanding brackets (the lesson before) and highest common factors from Year 7 Number.

The one idea: factorising divides every term by a common factor, and the factor goes outside the bracket. The common mistake is taking out a common factor that is not the highest (12x − 8 = 2(6x − 4)); it is planted after the method, in Ava's working. Her answer is equal to 12x − 8, so it is not wrong, just not finished: that is the conversation to have.

Answers are marked by form: an equal answer that could still be factorised further is told so ("could take out more") rather than marked right.

Running the lesson

  1. Predict: take a show of hands. Students who chose 6(x + 9) divided only the first term; those who chose 2(3x + 4.5) found a factor of 6 but not of 9. Do not correct it yet.
  2. Run: ask why 2 rows do not work (9 ones do not share into 2) and why 3 does.
  3. Investigate: model the worked example live, including the check by expanding. Then ask: is Ava's answer wrong?
  4. Modify: before the letter task, ask what goes into both 2x² and 4x. Students often find the 2 and miss the x.
  5. Make: collect students' expressions on the board and ask which ones are fully factorised with 4 outside.

When students get stuck

What you will seeA question to askThe fix
A student divides only the first term (6x + 9 = 3(2x + 9)).Expand your answer. Do you get back to where you started?Expanding is the check: 3 × 9 = 27, not 9.
A student stops at a common factor that is not the highest (Ava's mistake).Look inside the bracket. Do those terms share anything?Take out what they share as well: 2(6x − 4) = 2 × 2(3x − 2) = 4(3x − 2).
A student cannot find a common factor.List the factors of each number. Which appear in both lists?Use the board: try k = 2, 3, 4… and see which numbers of rows work.

Answers to the checks

  1. fully factorised: Which is 6x+96x + 9 written as a bracket, with the biggest possible number outside?
    3(2x + 3). 3 goes into 6 and into 9: 6x ÷ 3 = 2x and 9 ÷ 3 = 3. Check by expanding: 3 × 2x = 6x and 3 × 3 = 9.
  2. share into rows: The board has 6x+96x + 9. Change kk, the number of rows, until the tiles share into equal rows with more than one row. Was your prediction right?
  3. Take out the HCF: Factorise 10x+1510x + 15 fully.
    5(2x + 3)
    The HCF of 10 and 15 is 5: 10x ÷ 5 = 2x and 15 ÷ 5 = 3. Check: 5 × 2x + 5 × 3 = 10x + 15.
  4. Find the mistake: Ava factorises 12x−812x - 8. Click the line where her working goes wrong.
    line 2
    Line 2: 2 goes into both, but 4 is the biggest. 6x − 4 still shares a 2, so it is not fully factorised: 12x − 8 = 4(3x − 2).
  5. Check it: How can you check that 4(3x−2)4(3x - 2) is right for 12x−812x - 8?
    Expand it and compare
    Expand: 4 × 3x = 12x and 4 × (−2) = −8. You are back to 12x − 8.
  6. On the board: The board has 10x+1510x + 15. Share it into the most rows that work.

    5 rows of 2x + 3: 10x + 15 = 5(2x + 3).
  7. A minus sign: Factorise 9x−69x - 6 fully.
    3(3x - 2)
    HCF 3: 9x ÷ 3 = 3x and −6 ÷ 3 = −2. Check: 3(3x − 2) = 9x − 6.
  8. Three terms: Three terms: factorise 4x+8y−124x + 8y - 12 fully.
    4(x + 2y - 3)
    4 goes into 4, 8 and 12: 4(x + 2y − 3).
  9. A letter in common: A letter can be a common factor too. Factorise 2x2+4x2x^2 + 4x fully.
    2x(x + 2)
    2 and x go into both: 2x² ÷ 2x = x and 4x ÷ 2x = 2, so 2x(x + 2).
  10. Nothing to take out: Edge case: can 7x+57x + 5 be factorised?
    No: no number bigger than 1 goes into 7 and 5
    The HCF of 7 and 5 is 1, so 7x + 5 is already as simple as it gets. On the board, only 1 row works.
  11. Positive: Factorise 15x+2015x + 20 fully.
    5(3x + 4)
    HCF 5: 5(3x + 4).
  12. A minus sign: Factorise 8x−128x - 12 fully.
    4(2x - 3)
    HCF 4: 4(2x − 3).
  13. Full working: Factorise 18x+1218x + 12 fully. You may take out a factor in steps. Write one step per line.
    18x + 12 → 2(9x + 6) → 6(3x + 2)
    2(9x + 6) still shares a 3, so take it out too: 6(3x + 2). Or take out the HCF 6 straight away.
  14. Review: expanding: Review: expand 3(2x−5)3(2x - 5).
    6x - 15
    3 × 2x = 6x and 3 × (−5) = −15.
  15. four rows: Set ss and tt so that sx+tsx + t is fully factorised with 4 outside the bracket, then set kk to 4.

    8x + 12 = 4(2x + 3), 4x + 8 = 4(x + 2) and 12x + 4 = 4(3x + 1) all work. 8x + 8 does not: its HCF is 8.

Solutions

One possible solution for each task. Other correct programs are fine.

one change at a time › On the board

Open task: any program that does what the step asks.

one change at a time › A minus sign

Open task: any program that does what the step asks.

one change at a time › Three terms

Open task: any program that does what the step asks.

one change at a time › A letter in common

Open task: any program that does what the step asks.

one change at a time › Nothing to take out

Open task: any program that does what the step asks.

four rows

Open task: any program that does what the step asks.

Stretch and support

Stretch: Factorise 6x²y − 9xy² fully. (3xy(2x − 3y).) Then: write three different expressions whose full factorisation has 6 outside.

Support: Use the tiles board for every two-term question with positive numbers: try each k and read off the bracket.

Coach prompt (optional)

For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.

You are a patient maths coach for a Year 8 student in Australia. Help me understand ONE idea only, from the lesson "Factorising".

The idea: Factorising linear expressions: I can Students factorise linear expressions by identifying the highest common factor and express as a product of the HCF and a bracket.

What understanding looks like:
- Identifies the HCF of all terms in a linear expression.
- Writes the factorised form correctly.
- Verifies by expanding back to the original expression.

Mistakes students often make here:
- A student divides only the first term (6x + 9 = 3(2x + 9)).
- A student stops at a common factor that is not the highest (Ava's mistake).
- A student cannot find a common factor.

How to coach me (rules from the Waypoints Kit coach):
- Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question.
- Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit).
- If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go.
- If I seem to be missing the idea that comes before this one (Expanding and simplifying linear expressions), check it quickly, then come back.
- Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back.
- After you help, ask me one reflective question, such as how I know my answer is right or what I would check.
- Praise my effort and method, not my ability.
- You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so.
- Use simple words.
- Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Factorising".

Exit ticket

Factorise 14x − 21 fully, then check by expanding.

7(2x − 3). Check: 7 × 2x = 14x and 7 × (−3) = −21.

What each step is for

Where it leads

Next: rearranging formulas (node 3), which divides every term by a number in the same way.

Where it leads: factorising quadratics in Years 9 and 10, which the same tiles show as a rectangle.