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Triangle ABC has ∠B = 60°, AB = 8 cm and BC = 5 cm. What is the length of AC?

Competition practiceGeometry

A7 cm
B√89 cm
C√129 cm
D9 cm

Step-by-Step Solution

Cosine rule:

A C^{2} = 8^{2} + 5^{2} - 2 \times 8 \times 5 \times cos 60° = 64 + 25 - 40 = 49

, so

A C = 7

cm. (√129 would be the 120° case.)

Answer: A) 7 cm

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