cho $x_{1 }, x_{2},..., x_{n}$ >0,Chứng minh rằng:
$\dfrac{ x_{1} }{ x_{2}+2 x_{3}+...+(n-1) x_{n} } + \dfrac{ x_{2} }{ x_{3}+2 x_{4}+...+(n-1) x_{1} } +...+ \dfrac{ x_{n} }{x_{1}+2x_{n2}+...+(n-1) x_{n-1} }\geq$ $\dfrac{2}{n-1} $
@vietfrog: Gõ latex bạn nha!