Free Grade 3 Triangles Lesson Plans
Free Grade 3 lesson plans for Triangles: 12 real questions with a full answer key and worked solutions. Angles on a straight line add to 180°, around a point to 360°, and inside a triangle to 180°. A polygon with n sides has interior angles adding to (n − 2) × 180°.
Free
Higher
35 min
24
Learning objectives
Every objective is checkable, and each one maps to an activity and an assessment.
- RememberIdentify triangles
Identify triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
- UnderstandExplain triangles
Explain triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate triangles
Calculate triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare triangles
Compare triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
Prerequisites
Close these gaps first — each has its own free lesson, worksheet and quiz.
How Triangles works
Angles on a straight line add to 180°, around a point to 360°, and inside a triangle to 180°. A polygon with n sides has interior angles adding to (n − 2) × 180°.
12 practice questions
Easy to hard, 24 marks in total. Try them first, then open the answers.
- 1.Two angles on a straight line. One is 28°. Find the other.[1]
- 2.Two angles on a straight line. One is 78°. Find the other.[1]
- 3.Two angles on a straight line. One is 81°. Find the other.[1]
- 4.Two angles on a straight line. One is 136°. Find the other.[1]
- 5.A triangle has angles 89° and 57°. Find the third angle.[2]
- 6.A triangle has angles 83° and 38°. Find the third angle.[2]
- 7.A triangle has angles 35° and 33°. Find the third angle.[2]
- 8.A triangle has angles 26° and 130°. Find the third angle.[2]
- 9.Find the size of each interior angle of a regular 10-sided polygon.[3]
- 10.Find the size of each interior angle of a regular 8-sided polygon.[3]
- 11.Find the size of each interior angle of a regular 9-sided polygon.[3]
- 12.Find the size of each interior angle of a regular 7-sided polygon.[3]
Show answers and working
1. 152°
Angles on a straight line add to 180°. 180 − 28 = 152°.
2. 102°
Angles on a straight line add to 180°. 180 − 78 = 102°.
3. 99°
Angles on a straight line add to 180°. 180 − 81 = 99°.
4. 44°
Angles on a straight line add to 180°. 180 − 136 = 44°.
5. 34°
Angles in a triangle add to 180°. 180 − 89 − 57 = 34°.
6. 59°
Angles in a triangle add to 180°. 180 − 83 − 38 = 59°.
7. 112°
Angles in a triangle add to 180°. 180 − 35 − 33 = 112°.
8. 24°
Angles in a triangle add to 180°. 180 − 26 − 130 = 24°.
9. 144°
Sum of interior angles = (10 − 2) × 180 = 1440°. Each angle = 1440 ÷ 10 = 144°.
10. 135°
Sum of interior angles = (8 − 2) × 180 = 1080°. Each angle = 1080 ÷ 8 = 135°.
11. 140°
Sum of interior angles = (9 − 2) × 180 = 1260°. Each angle = 1260 ÷ 9 = 140°.
12. 128.57°
Sum of interior angles = (7 − 2) × 180 = 900°. Each angle = 900 ÷ 7 = 128.57°.
Worked examples
Two angles on a straight line. One is 83°. Find the other.
- Angles on a straight line add to 180°.
- 180 − 83 = 97°.
- Answer: 97°
A triangle has angles 36° and 41°. Find the third angle.
- Angles in a triangle add to 180°.
- 180 − 36 − 41 = 103°.
- Answer: 103°
Find the size of each interior angle of a regular 6-sided polygon.
- Sum of interior angles = (6 − 2) × 180 = 720°.
- Each angle = 720 ÷ 6 = 120°.
- Answer: 120°
Interactive practice
Free, no account required — answer on screen or print it.
Common mistakes
What learners typically get wrong, and how to fix it.
Reads the wrong scale on a protractor.
Correction: Start from the 0 on the arm you are measuring from.
Uses 360° for a triangle.
Correction: Triangle = 180°, quadrilateral = 360°.
Teacher tips
- · Estimate the angle type before measuring.
Parent tips
- · Spot right angles around the house.
Real-life applications
- · Ramps, clock hands, building and design.
Assessment objectives
How mastery is evidenced, and which standards it maps to.
Every free resource for Triangles
One concept, one ecosystem — all of it free.
Goes deeper
The entities below this one, each with the same complete free resource set.
Sibling concepts
Related concepts
Lateral links, including across subjects.
Next topics
All free formats for this concept
Every kind below is a real page, generated from this entity.
Learn — Meet the idea.
Practice — Build fluency.
Revise — Keep it in memory.
Assess — Prove mastery.
Apply — Use it in context.
Teach — Deliver it to a class.
Frequently asked
- Is this Grade 3 Triangles lesson plan really free?
- Yes. Every lesson plans on FreeMathWorksheet is free to use, print and share — no account, no paywall and no watermark.
- What should a learner already know before this?
- Start with Angles. Each one has its own free lesson, worksheet and quiz.
- How is the lesson plan sequenced?
- Recognition first, then understanding, application, reasoning and finally mastery — the same ladder used across every free resource on the platform.
- What comes next after Triangles?
- Move on to Quadrilaterals, Circles, Polygons, Symmetry, which build directly on this idea.
Related resources
Stuck on Triangles? The AI Tutor already knows this page, its prerequisites and its common mistakes.
Open the free AI Tutor