Two sides of a triangle are 7 cm and 8 cm with a 60° angle between them. What is the third side?
Answer and explanation
Correct answer: B. √57 cm
- Step 1: Cosine rule: c² = 49 + 64 − 2·7·8·cos 60°. Step 2. = 113 − 56 = 57, so c = √57. Answer: √57 cm. Why not the others: √113 omits the cosine term, √15 is a subtraction error and 9 is a guess from adding sides.
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