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By the cosine rule, cos γ in terms of the sides is
A(a² + b² + c²)/2ab
B(a² + c² − b²)/2ab
C(c² − a² − b²)/2ab
D(a² + b² − c²)/2ab
Answer & Solution
Correct answer: D. (a² + b² − c²)/2ab
Rearranging c² = a² + b² − 2ab cos γ gives cos γ = (a² + b² − c²)/2ab.
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