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定理1在任意△ABC中,A、B、C表示其三内角,则cos3A+cos3B+cos3C≥38.(等号当且仅当△ABC为正三角形时成立)证明由三角恒等式cos3A+cos3B+cos3C=(2R+r)3-3s2r4R3-1(R、r、s为△ABC的外接圆半...
Theorem 1 In any △ABC, A, B, C represent its three internal angles, then cos3A + cos3B + cos3C ≥ 38. (The equal sign is established if and only if △ABC is an equilateral triangle.) Prove by the trigonometric identity cos3A+cos3B+cos3C=(2R+r)3-3s2r4R3-1 (R, r, s are the circumcircle half of △ABC...