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Lính mới
Cho các số thực dương $a$, $b$, $c$, $d$ thỏa mãn $a+b+c+d=4$. Chứng minh rằng:
$\frac{1}{ab}+\frac{1}{cd}\geq \frac{a^{2}+b^{2}+c^{2}+d^{2}}{2}$
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