Cho $a,b,c,d\in \mathbb{R}^+,abcd=1$, Tìm GTLN $\frac{1}{bc +cd +db+1}+\frac{1}{ac +cd +da+1}+\frac{1}{da +ab +db+1}+\frac{1}{ab +bc +ca+1}$
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Cho $a,b,c,d\in \mathbb{R}^+,abcd=1$, Tìm GTLN $\frac{1}{bc +cd +db+1}+\frac{1}{ac +cd +da+1}+\frac{1}{da +ab +db+1}+\frac{1}{ab +bc +ca+1}$
Ta có $\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\geq \sum \frac{1}{\sqrt{ab}}=\sqrt{d}(\sqrt{a}+\sqrt{b}+\sqrt{c})\Rightarrow ab+bc+ca\geq \frac{\sqrt{a}+\sqrt{b}+\sqrt{c}}{\sqrt{d}}\Rightarrow \frac{1}{1+ab+bc+ca}\leq \frac{\sqrt{d}}{\sqrt{a}+\sqrt{b}+\sqrt{c}+\sqrt{d}}$.
Thiết lập các BĐT tương tự rồi cộng vào ta có max=1
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