Free Grade 3 Sine Rule Lessons
Free Grade 3 lessons for Sine Rule: 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
Stretch
25 min
24
Learning objectives
Every objective is checkable, and each one maps to an activity and an assessment.
- UnderstandExplain sine rule
Explain sine rule accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate sine rule
Calculate sine rule accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare sine rule
Compare sine rule accurately in a familiar context.
Assessed by: Exit ticket of four short items
- EvaluateJustify sine rule
Justify sine rule 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 Sine Rule 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 8 cm and 15 cm. Find the hypotenuse.[1]
- 2.A right-angled triangle has shorter sides 9 cm and 12 cm. Find the hypotenuse.[1]
- 3.A right-angled triangle has shorter sides 6 cm and 8 cm. Find the hypotenuse.[1]
- 4.A right-angled triangle has shorter sides 5 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 17 cm and one side 8 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 8 cm and the hypotenuse is 17 cm. Find θ to 1 d.p.[3]
- 10.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]
- 11.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]
- 12.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]
Show answers and working
1. 17 cm
c² = a² + b² = 64 + 225 = 289. c = √289 = 17 cm.
2. 15 cm
c² = a² + b² = 81 + 144 = 225. c = √225 = 15 cm.
3. 10 cm
c² = a² + b² = 36 + 64 = 100. c = √100 = 10 cm.
4. 13 cm
c² = a² + b² = 25 + 144 = 169. c = √169 = 13 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. 15 cm
b² = c² − a² = 289 − 64 = 225. b = √225 = 15 cm.
8. 4 cm
b² = c² − a² = 25 − 9 = 16. b = √16 = 4 cm.
9. 28.1°
sin θ = opposite / hypotenuse = 8/17. θ = sin⁻¹(0.4706) = 28.1°.
10. 22.6°
sin θ = opposite / hypotenuse = 5/13. θ = sin⁻¹(0.3846) = 22.6°.
11. 36.9°
sin θ = opposite / hypotenuse = 6/10. θ = sin⁻¹(0.6) = 36.9°.
12. 36.9°
sin θ = opposite / hypotenuse = 9/15. 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 13 cm and one side 5 cm. Find the other side.
- b² = c² − a² = 169 − 25 = 144.
- b = √144 = 12 cm.
- Answer: 12 cm
In a right-angled triangle, the side opposite angle θ is 7 cm and the hypotenuse is 25 cm. Find θ to 1 d.p.
- sin θ = opposite / hypotenuse = 7/25.
- θ = sin⁻¹(0.28) = 16.3°.
- Answer: 16.3°
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
How mastery is evidenced, and which standards it maps to.
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Frequently asked
- Is this Grade 3 Sine Rule 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 Right-angled Trigonometry. 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 Sine Rule?
- Move on to Cosine Rule, Area of a Triangle, Right-angled Trigonometry, Trigonometric Graphs, which build directly on this idea.
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