Math lesson plan · 7th grade (Year 8 in England) · 45 minutes
Decide whether three lengths can make a triangle
Geometry
CCSS 7.G.A.2England: KS3 Geometry and measuresEngland: GCSE Foundation Geometry and measures
Objective and success criteria
Students will: Decide whether three lengths can make a triangle.
- 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: Classify triangles; angles in a triangle
Leads to: the next skill in this area.
Materials: Rulers and protractors, Squared or dotted paper, Shape cut-outs, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Classify triangles; angles in a triangle). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- A triangle has no equal sides. What type of triangle is it? (equilateral / isosceles / right-angled / scalene) → scalenePrompt: Equilateral: all sides equal. Isosceles: two equal. Scalene: none equal.
- An isosceles triangle has two equal base angles of 33° each. What is the third angle, in degrees? → 114Prompt: The three angles add to 180°.
- A triangle has exactly two sides equal. What type of triangle is it? (equilateral / right-angled / isosceles / scalene) → isoscelesPrompt: Equilateral: all sides equal. Isosceles: two equal. Scalene: none equal.
Hook · 5 min
- Show a photo of a building, a tile pattern or a road sign and ask what shapes and angles students can spot.
- Share the objective: Decide whether three lengths can make a triangle. 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: Can sticks of length 9 cm, 2 cm and 11 cm make a triangle?
- Step 1: 2 + 9 = 11 ≤ 11
- Step 2: No.
- Answer: No
- 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.
- Can sticks of length 7 cm, 3 cm and 5 cm make a triangle? (Yes / No) → YesPrompt: The two shorter sides must add to MORE than the longest side.
- Two sides of a triangle are 9 cm and 5 cm. The third side is a whole number of cm. What is the smallest it can be? → 5Prompt: The third side must be less than the sum of the other two…
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.
- Two sides of a triangle are 7 cm and 15 cm. The third side is a whole number of cm. What is the smallest it can be? → 9Prompt: The third side must be less than the sum of the other two…
- Can sticks of length 3 cm, 7 cm and 12 cm make a triangle? (Yes / No) → NoPrompt: The two shorter sides must add to MORE than the longest side.
Watch out for
- Thinking a square turned on its corner is no longer a square.
- Reading the wrong scale on a protractor.
Support and stretch
Support
- Review first: Classify triangles; angles in a triangle.
- Use concrete materials: rulers and protractors.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- 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-7.
Free lesson plan from eurekami.com/lesson-plan-builder