This key facts covers The Cosine Rule 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 2 of 5 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
The Cosine Rule
| To find a side | a² = b² + c² - 2bc cos A |
| To find an angle | cos A = (b² + c² - a²) / 2bc |
| Use when | SAS (2 sides + included angle) or SSS (all 3 sides) |
| Note | The angle A is BETWEEN sides b and c |
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 cos A
Cosine Rule
a² = b² + c² - 2bc cosA. Use for SAS or SSS. Like Pythagoras with a twist!