Free Grade 3 Working with Area of a Triangle Lesson Plans
Free Grade 3 lesson plans for Working with Area of a Triangle: 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
Core
15 min
24
Learning objectives
Every objective is checkable, and each one maps to an activity and an assessment.
- UnderstandExplain working with area of a triangle
Explain working with area of a triangle accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate working with area of a triangle
Calculate working with area of a triangle accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare working with area of a triangle
Compare working with area of a triangle accurately in a familiar context.
Assessed by: Exit ticket of four short items
- EvaluateJustify working with area of a triangle
Justify working with area of a triangle 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 Working with Area of a Triangle 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 74°. Find the other.[1]
- 2.Two angles on a straight line. One is 147°. Find the other.[1]
- 3.Two angles on a straight line. One is 72°. Find the other.[1]
- 4.Two angles on a straight line. One is 133°. Find the other.[1]
- 5.A triangle has angles 63° and 25°. Find the third angle.[2]
- 6.A triangle has angles 23° and 23°. Find the third angle.[2]
- 7.A triangle has angles 43° and 109°. Find the third angle.[2]
- 8.A triangle has angles 20° and 54°. 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 6-sided polygon.[3]
- 11.Find the size of each interior angle of a regular 7-sided polygon.[3]
- 12.Find the size of each interior angle of a regular 6-sided polygon.[3]
Show answers and working
1. 106°
Angles on a straight line add to 180°. 180 − 74 = 106°.
2. 33°
Angles on a straight line add to 180°. 180 − 147 = 33°.
3. 108°
Angles on a straight line add to 180°. 180 − 72 = 108°.
4. 47°
Angles on a straight line add to 180°. 180 − 133 = 47°.
5. 92°
Angles in a triangle add to 180°. 180 − 63 − 25 = 92°.
6. 134°
Angles in a triangle add to 180°. 180 − 23 − 23 = 134°.
7. 28°
Angles in a triangle add to 180°. 180 − 43 − 109 = 28°.
8. 106°
Angles in a triangle add to 180°. 180 − 20 − 54 = 106°.
9. 140°
Sum of interior angles = (9 − 2) × 180 = 1260°. Each angle = 1260 ÷ 9 = 140°.
10. 120°
Sum of interior angles = (6 − 2) × 180 = 720°. Each angle = 720 ÷ 6 = 120°.
11. 128.57°
Sum of interior angles = (7 − 2) × 180 = 900°. Each angle = 900 ÷ 7 = 128.57°.
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 77°. Find the other.
- Angles on a straight line add to 180°.
- 180 − 77 = 103°.
- Answer: 103°
A triangle has angles 25° and 48°. Find the third angle.
- Angles in a triangle add to 180°.
- 180 − 25 − 48 = 107°.
- Answer: 107°
Find the size of each interior angle of a regular 10-sided polygon.
- Sum of interior angles = (10 − 2) × 180 = 1440°.
- Each angle = 1440 ÷ 10 = 144°.
- Answer: 144°
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 Working with Area of a Triangle 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 Introducing Area of a Triangle, Cosine Rule. 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 Working with Area of a Triangle?
- Move on to Area of a Triangle in Context, Sine Rule, Cosine Rule, which build directly on this idea.
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