When to Use Sine Rule vs Cosine Rule
Choosing between sine rule and cosine rule is one of the most common decisions in IGCSE trigonometry. This guide gives you a clear decision framework.
Use Sine Rule When
You have a matching pair: a side and its opposite angle. If you know angle A and side a, or can find them, use the sine rule.
Tips
- Need: one complete side-angle pair
- Plus one more side or angle
- a/sinA = b/sinB
Use Cosine Rule When
You have two sides and the included angle (SAS), or all three sides (SSS). There is no matching pair for the sine rule.
Tips
- SAS: two sides + angle between them
- SSS: all three sides, find an angle
- a² = b² + c² − 2bc·cosA
Decision Flowchart
Step 1: Do I have a complete opposite pair (side + its angle)? If yes → Sine rule. If no → Do I have SAS or SSS? If yes → Cosine rule.
Tips
- Always check for pairs first
- Draw and label your triangle
- Write out what you know before choosing
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Is this relevant for the 2025 syllabus?
Yes, all content is aligned with the Cambridge IGCSE Mathematics 0580 syllabus for 2025-2027.
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