Question

Find the cosine of angle B.

Answer 1

$\boxed { \text {Cosine Rule : } a^2 = b^2 + c^2 - 2bc \cos(A) }$

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9² = 12² + 15² - 2 (12) (15) cos (B)

81 = 144 + 225 - 360 cos(B)

81 = 369 - 360 cos (B)

360 cos (B) = 369 - 81

360 cos (B) = 288

cos (B) = 0.8

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Answer: Cosine B = 0.8

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$\boxed { \text {Cosine Rule : } a^2 = b^2 + c^2 - 2bc \cos(A) }$

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12² = 15² + 9² - 2 (15)(9) cos (A)

144 = 225 + 81 - 270 Cos A

144 = 306 - 270 Cos A

270 Cos A = 162

Cos A = 3/5 or 0.6

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Answer: Cosine Angle A = 3/5