Free Grade 3 Introducing Triangles Lesson Plans
Free Grade 3 lesson plans for Introducing 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
Stretch
25 min
24
Learning objectives
Every objective is checkable, and each one maps to an activity and an assessment.
- UnderstandExplain introducing triangles
Explain introducing triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate introducing triangles
Calculate introducing triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare introducing triangles
Compare introducing triangles accurately in a familiar context.
Assessed by: Exit ticket of four short items
- EvaluateJustify introducing triangles
Justify introducing 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 Introducing 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 87°. Find the other.[1]
- 2.Two angles on a straight line. One is 53°. Find the other.[1]
- 3.Two angles on a straight line. One is 115°. Find the other.[1]
- 4.Two angles on a straight line. One is 153°. Find the other.[1]
- 5.A triangle has angles 73° and 55°. Find the third angle.[2]
- 6.A triangle has angles 81° and 21°. Find the third angle.[2]
- 7.A triangle has angles 85° and 47°. Find the third angle.[2]
- 8.A triangle has angles 83° and 64°. Find the third angle.[2]
- 9.Find the size of each interior angle of a regular 9-sided polygon.[3]
- 10.Find the size of each interior angle of a regular 10-sided polygon.[3]
- 11.Find the size of each interior angle of a regular 5-sided polygon.[3]
- 12.Find the size of each interior angle of a regular 6-sided polygon.[3]
Show answers and working
1. 93°
Angles on a straight line add to 180°. 180 − 87 = 93°.
2. 127°
Angles on a straight line add to 180°. 180 − 53 = 127°.
3. 65°
Angles on a straight line add to 180°. 180 − 115 = 65°.
4. 27°
Angles on a straight line add to 180°. 180 − 153 = 27°.
5. 52°
Angles in a triangle add to 180°. 180 − 73 − 55 = 52°.
6. 78°
Angles in a triangle add to 180°. 180 − 81 − 21 = 78°.
7. 48°
Angles in a triangle add to 180°. 180 − 85 − 47 = 48°.
8. 33°
Angles in a triangle add to 180°. 180 − 83 − 64 = 33°.
9. 140°
Sum of interior angles = (9 − 2) × 180 = 1260°. Each angle = 1260 ÷ 9 = 140°.
10. 144°
Sum of interior angles = (10 − 2) × 180 = 1440°. Each angle = 1440 ÷ 10 = 144°.
11. 108°
Sum of interior angles = (5 − 2) × 180 = 540°. Each angle = 540 ÷ 5 = 108°.
12. 120°
Sum of interior angles = (6 − 2) × 180 = 720°. Each angle = 720 ÷ 6 = 120°.
Worked examples
Two angles on a straight line. One is 132°. Find the other.
- Angles on a straight line add to 180°.
- 180 − 132 = 48°.
- Answer: 48°
A triangle has angles 89° and 44°. Find the third angle.
- Angles in a triangle add to 180°.
- 180 − 89 − 44 = 47°.
- Answer: 47°
Find the size of each interior angle of a regular 8-sided polygon.
- Sum of interior angles = (8 − 2) × 180 = 1080°.
- Each angle = 1080 ÷ 8 = 135°.
- Answer: 135°
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.
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Frequently asked
- Is this Grade 3 Introducing 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 Introducing Triangles?
- Move on to Working with Triangles, Triangles in Context, Quadrilaterals, Circles, which build directly on this idea.
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