Solve the Law of cosines: c squared is equal to A squared plus b squared minus 2 A b cosine cap c for cosine cap c .

Start with c² = A² + b² − 2Ab cos C. Subtract c² from both sides and add 2Ab cos C to both sides to get 2Ab cos C = A² + b² − c². Dividing by 2Ab gives cos C = (A² + b² − c²)/(2Ab), which is choice B.

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