In triangle ABC, AB = 7, AC = 8 and angle A = 60°. What is BC?
Answer and explanation
Correct answer: A. √57
- Step 1: Use the cosine rule: BC² = AB² + AC² − 2·AB·AC·cos A. Step 2. cos 60° = 1/2, so BC² = 113 − 56 = 57. Answer: √57. Why not the others: Leaving out or adding the cosine term changes the result.
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