This deep dive covers Method 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 3 of 5 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Method
1
Check: Do you have SAS or SSS? If yes, use cosine rule
2
For side: a² = b² + c² - 2bc cos A, then square root
3
For angle: cos A = (b² + c² - a²) / 2bc, then inverse cos
Practice questions for Cosine Rule
Which formula is the cosine rule for finding side a?
Show that when angle A = 90°, the cosine rule reduces to Pythagoras' theorem.
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