Free Grade 3 Introducing Pythagoras' Theorem Lessons
Free Grade 3 lessons for Introducing Pythagoras' Theorem: 12 real questions with a full answer key and worked solutions. In a right-angled triangle, a² + b² = c², where c is the hypotenuse (the longest side, opposite the right angle). Trigonometry links the sides to the angles.
Free
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35 min
24
Learning objectives
Every objective is checkable, and each one maps to an activity and an assessment.
- UnderstandExplain introducing pythagoras' theorem
Explain introducing pythagoras' theorem accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate introducing pythagoras' theorem
Calculate introducing pythagoras' theorem accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare introducing pythagoras' theorem
Compare introducing pythagoras' theorem accurately in a familiar context.
Assessed by: Exit ticket of four short items
- EvaluateJustify introducing pythagoras' theorem
Justify introducing pythagoras' theorem 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 Pythagoras' Theorem works
In a right-angled triangle, a² + b² = c², where c is the hypotenuse (the longest side, opposite the right angle). Trigonometry links the sides to the angles.
12 practice questions
Easy to hard, 24 marks in total. Try them first, then open the answers.
- 1.A right-angled triangle has shorter sides 7 cm and 24 cm. Find the hypotenuse.[1]
- 2.A right-angled triangle has shorter sides 5 cm and 12 cm. Find the hypotenuse.[1]
- 3.A right-angled triangle has shorter sides 8 cm and 15 cm. Find the hypotenuse.[1]
- 4.A right-angled triangle has shorter sides 9 cm and 12 cm. Find the hypotenuse.[1]
- 5.A right-angled triangle has hypotenuse 15 cm and one side 9 cm. Find the other side.[2]
- 6.A right-angled triangle has hypotenuse 25 cm and one side 7 cm. Find the other side.[2]
- 7.A right-angled triangle has hypotenuse 13 cm and one side 5 cm. Find the other side.[2]
- 8.A right-angled triangle has hypotenuse 5 cm and one side 3 cm. Find the other side.[2]
- 9.In a right-angled triangle, the side opposite angle θ is 9 cm and the hypotenuse is 15 cm. Find θ to 1 d.p.[3]
- 10.In a right-angled triangle, the side opposite angle θ is 6 cm and the hypotenuse is 10 cm. Find θ to 1 d.p.[3]
- 11.In a right-angled triangle, the side opposite angle θ is 5 cm and the hypotenuse is 13 cm. Find θ to 1 d.p.[3]
- 12.In a right-angled triangle, the side opposite angle θ is 3 cm and the hypotenuse is 5 cm. Find θ to 1 d.p.[3]
Show answers and working
1. 25 cm
c² = a² + b² = 49 + 576 = 625. c = √625 = 25 cm.
2. 13 cm
c² = a² + b² = 25 + 144 = 169. c = √169 = 13 cm.
3. 17 cm
c² = a² + b² = 64 + 225 = 289. c = √289 = 17 cm.
4. 15 cm
c² = a² + b² = 81 + 144 = 225. c = √225 = 15 cm.
5. 12 cm
b² = c² − a² = 225 − 81 = 144. b = √144 = 12 cm.
6. 24 cm
b² = c² − a² = 625 − 49 = 576. b = √576 = 24 cm.
7. 12 cm
b² = c² − a² = 169 − 25 = 144. b = √144 = 12 cm.
8. 4 cm
b² = c² − a² = 25 − 9 = 16. b = √16 = 4 cm.
9. 36.9°
sin θ = opposite / hypotenuse = 9/15. θ = sin⁻¹(0.6) = 36.9°.
10. 36.9°
sin θ = opposite / hypotenuse = 6/10. θ = sin⁻¹(0.6) = 36.9°.
11. 22.6°
sin θ = opposite / hypotenuse = 5/13. θ = sin⁻¹(0.3846) = 22.6°.
12. 36.9°
sin θ = opposite / hypotenuse = 3/5. θ = sin⁻¹(0.6) = 36.9°.
Worked examples
A right-angled triangle has shorter sides 3 cm and 4 cm. Find the hypotenuse.
- c² = a² + b² = 9 + 16 = 25.
- c = √25 = 5 cm.
- Answer: 5 cm
A right-angled triangle has hypotenuse 10 cm and one side 6 cm. Find the other side.
- b² = c² − a² = 100 − 36 = 64.
- b = √64 = 8 cm.
- Answer: 8 cm
In a right-angled triangle, the side opposite angle θ is 8 cm and the hypotenuse is 17 cm. Find θ to 1 d.p.
- sin θ = opposite / hypotenuse = 8/17.
- θ = sin⁻¹(0.4706) = 28.1°.
- Answer: 28.1°
Interactive practice
Free, no account required — answer on screen or print it.
Common mistakes
What learners typically get wrong, and how to fix it.
Adds the squares when finding a shorter side.
Correction: Shorter side: subtract from the hypotenuse squared.
Forgets to square root at the end.
Correction: c² is not c — finish with √.
Teacher tips
- · Label the hypotenuse first every time.
Parent tips
- · Measure a phone screen's sides and check the diagonal.
Real-life applications
- · Ladder length against a wall, TV screen sizes.
Assessment objectives
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Frequently asked
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- What should a learner already know before this?
- Start with Tangent Ratio. 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 Introducing Pythagoras' Theorem?
- Move on to Working with Pythagoras' Theorem, Pythagoras' Theorem in Context, Sine Ratio, Cosine Ratio, which build directly on this idea.
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