Cho tam giác ABC, tìm giá trị nhỏ nhất của $\text{S}=\large{\dfrac{\cos^{2}\dfrac{A}{2}\cos^{2}\dfrac{B}{2}}{\cos^{2}\dfrac{C}{2}}+\dfrac{\cos^{2}\dfrac{B}{2}\cos^{2}\dfrac{C}{2}}{\cos^{2}\dfrac{A}{2}} +\dfrac{\cos^{2}\dfrac{C}{2}\cos^{2}\dfrac{A}{2}}{\cos^{2}\dfrac{B}{2}}}$
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