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vipboycodon

$P = \dfrac{3y+9x}{xy}+\dfrac{26}{3x+y}$
$P = 3x+y+\dfrac{36}{3x+y}-\dfrac{10}{3x+y} \ge 2\sqrt{(3x+y).\dfrac{36}{3x+y}}-\dfrac{10}{2\sqrt{3xy}} = 12-\dfrac{10}{6} = \dfrac{31}{3}$
Dấu "=" xảy ra khi $x = 1$ , $y = 3$
 
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