Math lesson plan · 4th grade (Year 5 in England) · 60 minutes
Classify triangles by their angles
Geometry
CCSS 4.G.A.2England: Y5 Geometry
Objective and success criteria
Students will: Classify triangles by their angles.
- 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: Angles: types and measuring
Leads to: Classify triangles by their sides
Materials: Rulers and protractors, Squared or dotted paper, Shape cut-outs, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 6 min
- Quick retrieval from earlier learning (Angles: types and measuring). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- What type of angle is 79°? (acute / right / obtuse / straight / reflex) → acutePrompt: Compare with a right angle (90°) and a straight line (180°).
- Two angles together make a straight line. One is 31°. What is the other, in degrees? → 149Prompt: Angles on a straight line add up to 180°.
- A right angle is split into two parts. One part is 55°. What is the other part, in degrees? → 35Prompt: A right angle is 90°.
Hook · 6 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: Classify triangles by their angles. Read the success criteria together.
I do: model it · 12 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: A triangle has angles of 14°, 24°, 142°. What type of triangle is it?
- Step 1: Largest angle: 142°
- Step 2: obtuse-angled
- Answer: obtuse-angled
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 12 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- Hana says: "A right-angled triangle can also be isosceles." True or false? (true / false) → truePrompt: The angles of a triangle add up to 180°.
- Grace says: "A triangle can have two obtuse angles." True or false? (true / false) → falsePrompt: The angles of a triangle add up to 180°.
You do: independent practice · 17 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 · 7 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- A triangle has angles of 54°, 115°, 11°. What type of triangle is it? (acute-angled / right-angled / obtuse-angled) → obtuse-angledPrompt: Look at the largest angle.
- Arjun says: "A triangle can have two obtuse angles." True or false? (true / false) → falsePrompt: The angles of a triangle add up to 180°.
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: Angles: types and measuring.
- 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
- Move on to: Classify triangles by their sides.
- 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-4.
Free lesson plan from eurekami.com/lesson-plan-builder