Free Grade 3 Area of a Triangle in Context Lessons
Free Grade 3 lessons for Area of a Triangle in Context: 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 area of a triangle in context
Identify area of a triangle in context accurately in a familiar context.
Assessed by: Exit ticket of four short items
- UnderstandExplain area of a triangle in context
Explain area of a triangle in context accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate area of a triangle in context
Calculate area of a triangle in context accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare area of a triangle in context
Compare area of a triangle in context 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 Area of a Triangle in Context 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 59°. Find the other.[1]
- 2.Two angles on a straight line. One is 54°. Find the other.[1]
- 3.Two angles on a straight line. One is 84°. Find the other.[1]
- 4.Two angles on a straight line. One is 92°. Find the other.[1]
- 5.A triangle has angles 50° and 92°. Find the third angle.[2]
- 6.A triangle has angles 32° and 88°. Find the third angle.[2]
- 7.A triangle has angles 38° and 56°. Find the third angle.[2]
- 8.A triangle has angles 46° and 63°. Find the third angle.[2]
- 9.Find the size of each interior angle of a regular 8-sided polygon.[3]
- 10.Find the size of each interior angle of a regular 9-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 10-sided polygon.[3]
Show answers and working
1. 121°
Angles on a straight line add to 180°. 180 − 59 = 121°.
2. 126°
Angles on a straight line add to 180°. 180 − 54 = 126°.
3. 96°
Angles on a straight line add to 180°. 180 − 84 = 96°.
4. 88°
Angles on a straight line add to 180°. 180 − 92 = 88°.
5. 38°
Angles in a triangle add to 180°. 180 − 50 − 92 = 38°.
6. 60°
Angles in a triangle add to 180°. 180 − 32 − 88 = 60°.
7. 86°
Angles in a triangle add to 180°. 180 − 38 − 56 = 86°.
8. 71°
Angles in a triangle add to 180°. 180 − 46 − 63 = 71°.
9. 135°
Sum of interior angles = (8 − 2) × 180 = 1080°. Each angle = 1080 ÷ 8 = 135°.
10. 140°
Sum of interior angles = (9 − 2) × 180 = 1260°. Each angle = 1260 ÷ 9 = 140°.
11. 108°
Sum of interior angles = (5 − 2) × 180 = 540°. Each angle = 540 ÷ 5 = 108°.
12. 144°
Sum of interior angles = (10 − 2) × 180 = 1440°. Each angle = 1440 ÷ 10 = 144°.
Worked examples
Two angles on a straight line. One is 104°. Find the other.
- Angles on a straight line add to 180°.
- 180 − 104 = 76°.
- Answer: 76°
A triangle has angles 49° and 89°. Find the third angle.
- Angles in a triangle add to 180°.
- 180 − 49 − 89 = 42°.
- Answer: 42°
Find the size of each interior angle of a regular 7-sided polygon.
- Sum of interior angles = (7 − 2) × 180 = 900°.
- Each angle = 900 ÷ 7 = 128.57°.
- Answer: 128.57°
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 Area of a Triangle in Context lesson really free?
- Yes. Every lessons 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 Working with Area of a Triangle, Introducing Area of a Triangle, Cosine Rule. Each one has its own free lesson, worksheet and quiz.
- How is the lesson 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 Area of a Triangle in Context?
- Move on to Sine Rule, Cosine Rule, which build directly on this idea.
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