4MA1/2H Jun 2018, Question 18: The Cosine Rule
Pearson Edexcel International GCSE Mathematics A 4MA1 · Paper 2H (Higher) · Calculator · 3 marks
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The question, in short
Using sides 8 cm, 10 cm and 14 cm, calculate the triangle's largest angle, giving the result to 1 decimal place.
Summarised in our own words. Use the official paper for the exact wording and any diagrams.
Solution · 3 marks
- 1The largest angle A is opposite the longest side, 14 cm
- 2Cosine rule: cos A = (10² + 8² − 14²) ÷ (2 × 10 × 8)
- 3cos A = (100 + 64 − 196) ÷ 160 = −32 ÷ 160 = −0.2
- 4A = cos⁻¹(−0.2) = 101.5
Answer101.5
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