3 Sean $ x_{1},x_{2},\cdots,x_{m},y_{1},y_{2},\cdots,y_{n}$ números reales positivos. Denótese por $ X = \sum_{i = 1}^{m}x,Y = \sum_{j = 1}^{n}y.$ Demuestre que $ 2XY\sum_{i = 1}^{m}\sum_{j = 1}^{n}|x_{i} - y_{j}|\ge X^2\sum_{j = 1}^{n}\sum_{l = 1}^{n}|y_{i} - y_{l}| + Y^2\sum_{i = 1}^{m}\sum_{k = 1}^{m}|x_{i} - x_{k}|$ Fang-jh