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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...