In △ ABC, if a = 7, B = 8, COSC = 1314, then the cosine of the largest angle is () A. −15B. −16C. −17D. −18

∵ in △ ABC, a = 7, B = 8, COSC = 1314, ∵ C2 = A2 + b2-2abcosc = 49 + 64-2 × 7 × 8 × 1314 = 9, C = 3 ∵ b > a > C, ∵ the largest edge is B, and B is the largest angle. Therefore, CoSb = 49 + 9 − 642 × 7 × 3 = − 17, that is, the cosine value of the largest angle is − 17

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