考慮\(\sin \frac{x}{2}=\sqrt{\frac{1-\cos x}{2}}\)

也可以把 plan B 改成\(\frac{2{{\sin }^{2}}\left( \frac{x}{2} \right)+{{\cos }^{2}}\left( \frac{x}{2} \right)+{{\cos }^{2}}\left( \frac{x}{2} \right)}{3}\ge \sqrt[3]{2{{\sin }^{2}}\left( \frac{x}{2} \right){{\cos }^{2}}\left( \frac{x}{2} \right){{\cos }^{2}}\left( \frac{x}{2} \right)}\)