Find side a when b = 7cm, c = 9cm, and A = 60°.
Solution
a² = b² + c² - 2bc cos A
a² = 49 + 81 - 2(7)(9)cos 60°
a² = 130 - 126(0.5) = 130 - 63 = 67
a = √67 = 8.2 cm (1 d.p.)
This study notes covers Worked Example: Finding a Side 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 4 of 5 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
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!