You will learn
- Recognise a triangle.
- Say that a triangle has 3 sides and 3 corners.
- Tell a triangle apart from other shapes.
- Find triangles around us.
Version 0.2 · expanded edition · built from cited public sources Content is based on the official frameworks and sources listed under Sources and standards. No official endorsement is claimed. Written in the style of common primary school assessments. No exam result is promised.
Student lesson
- A triangle has 3 straight sides.
- A triangle has 3 corners.
- It must be closed, with no gaps.
- A triangle can be turned and is still a triangle.
Words to know
A shape with 3 straight sides and 3 corners.
A straight line that makes the edge of a shape.
Where two sides meet.
The number 3.
1. Discover: a roof and a slice
Look at the roof of a tent or a slice of cake cut from the middle. What shape do they remind you of?
2. Understand: what is a triangle?
A triangle has 3 straight sides and 3 corners. The sides are closed: there are no gaps.
Triangles can look different. Some are tall, some are wide, and some are tilted. They are all triangles if they have 3 straight sides and 3 corners.
Idea 1: Three sides and three corners
A triangle has 3 straight sides and 3 corners. Trace one with your finger: 3 sides, 3 corners. The word part tri means three.
Idea 2: A triangle is closed
All the sides of a triangle join up. If there is a gap, it is not a triangle.
Idea 3: Triangles can turn
A triangle can be big or small, tilted or upside down. It is still a triangle because it still has 3 straight sides and 3 corners.
3. Worked examples
Example 1 (I show you): Count sides and corners
How many sides and corners has a triangle?
- Trace the edge with your finger.
- Count the straight sides: 3.
- Count the corners: 3.
Answer: 3 sides and 3 corners.
Example 2 (We do it together): Closed or open?
A shape has 3 straight lines, but two of them do not join. Is it a triangle?
- A triangle must be closed.
- There is a gap.
Answer: No. It is not a triangle.
Example 3 (You try): Triangles around us
Name something shaped like a triangle.
Show the steps and answer
- Think of a slice of pizza, a roof or a road sign.
- Check: 3 straight sides and 3 corners.
Answer: A roof end or a pizza slice.
Did you know?
The word triangle means three angles. The part tri means three.
Why this matters
Triangles are strong shapes. Builders use them in roofs and bridges.
4. Explore: make triangles
You need 3 sticks or straws and some clay (or draw lines).
- Lay 3 sticks end to end to make a closed shape.
- Count the sides and corners.
- Try with 4 sticks. Is it a triangle?
- Draw 3 different triangles.
Think about it: what did you notice?
3 sides make a triangle. 4 sides make a different shape.
5. Practise
Say it in a sentence
Use these starters when you talk about the lesson. Say them out loud.
- A triangle has ___ sides.
- A triangle has ___ corners.
- ___ is shaped like a triangle.
- A shape with a gap is not a ___.
Guided practice
Question 1. How many sides does a triangle have?
3.
Question 2. How many corners?
3.
Question 3. Is a shape with 4 sides a triangle?
No.
Independent practice
Level 1: Start
- Draw a triangle.
- How many sides has a triangle?
- How many corners has a triangle?
- Name something shaped like a triangle.
- Can a triangle have a curved side?
Level 2: Grow
- How many sides and corners does a triangle have?
- Is a square a triangle?
- Name something shaped like a triangle.
Level 3: Stretch
- Draw a big triangle and a small triangle. Are they both triangles?
- A shape has 3 sides but one is curved. Is it a triangle?
- Turn a triangle upside down. How many sides does it have now?
Answers are in the answer key below. Try first, then check.
6. Apply: everyday mathematics
A tent is shaped like a triangle on its front. How many corners does the front have?
Show a sample answer
3 corners.
Another one. The front of a roof has 3 straight sides. What shape is it?
Show a sample answer
A triangle.
7. Common mistakes
Fix: A triangle can be turned any way. Count 3 straight sides and 3 corners.
Fix: Turning a shape does not change it. It still has 3 sides and 3 corners.
Fix: A triangle must be closed. All the sides must join.
8. Lesson summary and revision card
- I can recognise a triangle.
- I can say a triangle has 3 sides and 3 corners.
- I can tell a triangle from other shapes.
- I can find triangles around me.
9. Check and master
Answer all 14 questions. You will see feedback after each one.
Your result appears here after you answer every question.
10. Key takeaways
- A triangle has 3 sides and 3 corners.
- Triangles can look different.
- Turning a triangle keeps it a triangle.
11. Next lesson
Next lesson: Squares. Keep going with the next idea in this course.
Exam practice
These questions are written in the style of common school and international primary assessments. Try them on your own, without looking back. Then check your answers in the marking guide. The marks are shown in brackets.
- How many sides does a triangle have? [1 mark]
- How many corners does a triangle have? [1 mark]
- Name two objects shaped like a triangle. [2 marks]
- A shape has 3 straight sides but there is a gap between two of them. Is it a triangle? Explain. [2 marks]
- Draw a triangle. [2 marks]
Total: 8 marks.
Marking guide and model answers
- 3 [1]
- 3 [1]
- Any two correct [1 each]
- No [1]. A triangle must be closed [1].
- Three straight sides [1]; closed with 3 corners [1]
Score guide: 7 to 8 = secure. 5 to 6 = nearly there, revise the parts you missed. Under 5 = work through the lesson again with a grown-up.
Printable worksheet
- Sides of a triangle?
- Corners of a triangle?
- Name something shaped like a triangle.
- Draw a triangle.
- Can a triangle have 4 sides? (yes/no)
- How many sides has a triangle?
- How many corners has a triangle?
- Is a circle a triangle?
- Name something shaped like a triangle.
- Cut a square from corner to corner. What shapes do you get?
Homework
Find or draw five triangles at home or in your neighbourhood. Count the sides and corners.
Answer key and marking scheme
| Question | Expected response | Marks |
|---|---|---|
| Sides of a triangle? | 3 | 2 |
| Corners of a triangle? | 3 | 2 |
| Name something shaped like a triangle. | Any: roof, slice, flag, tent | 2 |
| Draw a triangle. | Closed shape with 3 straight sides | 2 |
| Can a triangle have 4 sides? (yes/no) | no | 2 |
| Total | 10 |
Practice answers: Worksheet: 1 3; 2 3; 3 Any: roof, slice, flag, tent; 4 Closed shape with 3 straight sides; 5 no.
Quiz answers: 1 A, 2 B, 3 C, 4 A, 5 B, 6 A, 7 B, 8 C, 9 A, 10 B, 11 C, 12 A, Challenge 1 C, Challenge 2 C.
Extra worksheet answers (items 6 onward): 6 3; 7 3; 8 No; 9 A roof or a pizza slice; 10 Two triangles.
Independent practice answers. Level 1: 1 a closed shape with 3 straight sides; 2 3; 3 3; 4 for example a slice of pizza or a roof; 5 no, a triangle has only straight sides. Level 2: 1 3 sides and 3 corners; 2 No. A square has 4 sides.; 3 A slice of pizza, a roof or a road sign. Level 3: 1 Yes. Size does not change the shape.; 2 No. All the sides of a triangle are straight.; 3 Still 3 sides..
Mastery and remediation
| Objective | Bronze (Developing) | Silver (Proficient) | Gold (Advanced) |
|---|---|---|---|
| O1 Recognise a triangle | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
| O2 Say that a triangle has 3 sides and 3 corners | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
| O3 Tell a triangle apart from other shapes | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
| O4 Find triangles around us | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
Courseversity badge thresholds (overall quiz score): Gold = 90–100%; Silver = 80–89.9%; Bronze = 70–79.9%. A badge is never a full record of progress.
If a learner struggles
- Find the misconception: Thinking a triangle must point up. A triangle can be turned any way. Count 3 straight sides and 3 corners.
- Explain another way: use real objects first, then pictures.
- Show a simpler example: Trace a triangle with a finger, counting 3 sides and 3 corners.
- Guided practice: work through one example together and let the learner try the next one.
- Reassess: use the quiz again or the worksheet.
- Move on when the learner shows Silver-level evidence independently.
For parents and guardians
In this lesson your child learns to recognise a triangle. You do not need to be good at mathematics to help.
- Look for triangles at home: a folded cloth, a roof, a road sign.
- Draw triangles of different sizes.
- Fold a square of paper corner to corner and see the triangles.
- Ask “how do you know?” after each answer, and listen.
- Praise effort, not only right answers. Mistakes are how we learn.
- Ten to fifteen minutes a day is better than one long session.
Teacher package
1–5. Objectives, prerequisites, materials, preparation, duration
Lesson objectives. Learners will: recognise a triangle; say that a triangle has 3 sides and 3 corners; tell a triangle apart from other shapes; find triangles around us.
Prerequisite knowledge. Lesson 1: circles.
Materials. Counters (stones, seeds or bottle tops), paper, pencils, board or chalkboard. Low-resource options: use stones, seeds, sticks, bottle tops or drawings in the sand or on the board instead of printed materials.
Preparation. Gather the counters and draw the example pictures on the board before the lesson.
Suggested duration. 45–60 minutes. It can be split into two shorter sessions.
6–9. Teaching procedure, explanation notes, guided and independent activities
Introduction (5 min). Start with the Discover situation and let learners suggest ideas before you teach.
Explanation (10 min). Teach the Understand section with real objects first, then pictures, then symbols and words.
Guided activity (10 min). You need 3 sticks or straws and some clay (or draw lines). Learners follow the Explore steps in pairs.
Independent activity (10 min). Learners complete the independent practice and the worksheet.
Assessment and reflection (5 min). Ask each learner to say one thing they learned and how they know it.
Key questions. How many sides does a triangle have? Must it be closed? Is a tilted triangle still a triangle?
Teacher explanation notes. Use the concrete, pictorial, abstract order: real objects first, then pictures, then symbols and words. Return to real objects whenever a learner needs support.
10–12. Differentiation, formative assessment, common misconceptions
Support. Use real objects and work with a partner; say each step aloud together. Extension. Ask learners to make up their own example and explain it to a partner.
Formative assessment. Listen to learner explanations, ask “how do you know?”, and check the independent work.
Misconceptions. Thinking a triangle must point up. A triangle can be turned any way. Count 3 straight sides and 3 corners.
13–15. Assessment questions, answer keys, marking scheme
Use the quiz and worksheet on this page. The 8-mark exam practice scheme, the 10-mark scheme and all answers are in the answer sections above. Re-check every answer and the marking scheme before publication.
16–19. Remediation, enrichment, homework, teacher reflection
Remediation. Trace a triangle with a finger, counting 3 sides and 3 corners. Enrichment. Invite learners to find or create three more examples at home or in the classroom and explain them. Homework. See above.
Teacher reflection. After teaching, record: which concepts learners understood; which misconceptions occurred; which activities worked; which learners need more practice; and what to improve next time.
Sources and standards
This lesson is based on the sources below, and no official endorsement is claimed. Reference starting points named in the Courseversity standard: Common Core State Standards for Mathematics, Cambridge Primary and the IB Primary Years Programme. Where a standard code is named, it points to the official framework.
Official standards for this topic area. Common Core State Standards for Mathematics (official site): Grade 1 » Geometry. These pages list the standards that cover this topic area; read them alongside the lesson.
Research used to shape this lesson. Common Core 1.G.A.1 (defining attributes of shapes); Cambridge Primary Mathematics Stage 1. These sources informed the lesson.