Math lesson plan · 9th grade (Year 10 in England) · 45 minutes

Difference of two squares

Algebra

CCSS HSA.SSE.A.2England: GCSE Foundation AlgebraEngland: GCSE Higher Algebra

Objective and success criteria

Students will: Difference of two squares.

I can answer questions on today's skill on my own, without help.
I can explain each step and say why it works.
I can check my answer and spot a mistake.

Before and after

Needs first: Factorise quadratics x² + bx + c; Square roots and cube roots

Leads to: Factorise fully: common factor first, then the bracket; Simplify algebraic fractions by factorising; Factorise sums and differences of cubes

Materials: Balance scales or a balance drawing, Algebra tiles, Function machine cards, Mini whiteboards and pens, Printed student sheet (below).

Warm-up · 5 min

Quick retrieval from earlier learning (Factorise quadratics x² + bx + c; Square roots and cube roots). Students answer on mini whiteboards.
Show answers together and note who needs support in the main task.

Factorise: x² − 14x + 45 ((x + 45)(x + 1) / (x + 9)(x + 5) / (x − 9)(x + 5) / (x − 9)(x − 5)) → (x − 9)(x − 5)Prompt: Find two numbers that multiply to 45 and add to −14.
What is √144? → 12Prompt: Which number multiplied by itself gives this?
Factorise fully: x² − 3x − 4 → (x-4)(x+1)Prompt: Two numbers: product −4, sum −3.

Hook · 5 min

Show a "think of a number" puzzle and ask the class to work out the starting number.
Share the objective: Difference of two squares. Read the success criteria together.

I do: model it · 10 min

Work through the example out loud, saying why you do each step, not just what you do.
Example: Factorise: 9x² − 16
Step 1: (3x + 4)(3x − 4)
Answer: (3x + 4)(3x − 4)
Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 min

Do these together. Students try each step on whiteboards before you reveal it.
Use the hints as prompts rather than giving the answer.

Factorise: x² − 121 ((x + 11)² / (x − 11)² / (x + 121)(x − 1) / (x + 11)(x − 11)) → (x + 11)(x − 11)Prompt: This is a difference of two squares: A² − B² = (A + B)(A − B).
Use the difference of two squares to work out 52² − 48² without a calculator. → 400Prompt: a² − b² = (a + b)(a − b).

You do: independent practice · 10 min

Students work on the student sheet. Circulate and check the first two answers of every student early.
Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

Factorise: 25x² − 36 ((5x − 6)² / (5x + 6)(5x − 6) / 5(x + 6)(x − 6) / (5x + 6)²) → (5x + 6)(5x − 6)Prompt: This is a difference of two squares: A² − B² = (A + B)(A − B).
Use the difference of two squares to work out 42² − 38² without a calculator. → 320Prompt: a² − b² = (a + b)(a − b).

Watch out for

Doing an operation to one side of an equation but not the other.
Reading 3x as 3 + x, or thinking x always stands for one particular number.

Support and stretch

Support

Review first: Factorise quadratics x² + bx + c.
Use concrete materials: balance scales or a balance drawing.
Give the first step of each question, or a partly worked example to finish.
Pair with a confident partner for the guided practice.

Stretch

Move on to: Factorise fully: common factor first, then the bracket; Simplify algebraic fractions by factorising; Factorise sums and differences of cubes.
Explain a common mistake in writing and how to fix it.
Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-9.

Free lesson plan from eurekami.com/lesson-plan-builder