Tìm Max

M

minhtuyb

Ta chỉ xét trường hợp $x,y>0$
Từ gt ta có: $x\ge xy+1\ge 2\sqrt{xy}\Rightarrow x\ge 4y\Leftrightarrow \dfrac{x}{y}\ge 4$
$P$ max $\Leftrightarrow \dfrac{1}{P}$ min:
$$\dfrac{1}{P}=\dfrac{x}{3y}+\dfrac{y}{3x} \\ =(\dfrac{x}{48y}+\dfrac{y}{3x})+\dfrac{5}{16}.\dfrac{x}{y}\\ \ge 2\sqrt{\dfrac{x}{48y}.\dfrac{y}{3x}}+\dfrac{5}{16}.4\\ =\dfrac{12}{17}\\ \Rightarrow P\le \dfrac{17}{12}\ (const)$$
Dấu bằng xảy ra khi $x=2;y=\dfrac{1}{2}\ \square$
 
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