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Geometry & MeasuresIntroduction

When Sine Rule Fails

Part of Cosine Rule — GCSE Mathematics

This introduction covers When Sine Rule Fails within Cosine Rule for GCSE Mathematics. Revise Cosine Rule in Geometry & Measures for GCSE Mathematics with 12 exam-style questions and 3 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 1 of 4 in this topic. Use this introduction to connect the idea to the wider topic before moving on to questions and flashcards.

Topic position

Section 1 of 4

Practice

12 questions

Recall

3 flashcards

When Sine Rule Fails

Sometimes you know two sides and the angle BETWEEN them (SAS), or all three sides (SSS) but no angles. The sine rule can't help - it needs a matched pair! That's when you reach for the cosine rule.
Cosine rule diagram

Keep building this topic

Read this section alongside the surrounding pages in Cosine Rule. That gives you the full topic sequence instead of a single isolated revision point.

The Cosine RuleMethodWorked Example: Finding a Side

Practice Questions for Cosine Rule

Which formula is the cosine rule for finding side a?

  • A. a² = b² + c² − 2bc cosA
  • B. a/sinA = b/sinB
  • C. a² = b² + c²
  • D. cosA = (b + c − a) / 2bc
1 markfoundation

Show that when angle A = 90°, the cosine rule reduces to Pythagoras' theorem.

2 marksstandard

Quick Recall Flashcards

Cosine Rule
a² = b² + c² - 2bc cosA. Use for SAS or SSS. Like Pythagoras with a twist!
Cosine Rule
a² = b² + c² - 2bc cos A

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The Cosine Rule