Cho $x,y\in \left [ 0,1 \right ]$. Chứng minh rằng:
$$\sqrt{1+x^2}+\sqrt{1+y^2}+\sqrt{\left ( 1-x \right )^2+\left ( 1-y \right )^2}\geq \left ( 1+\sqrt{5} \right )\left ( 1-xy \right )$$
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