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[Laxus]

$x\sqrt{12-x}+(11-x)\sqrt{x+1}\geq 25$

[Basara]

ĐK $-1\leq x\leq 12$

đặt $t=\sqrt{12-x}+\sqrt{x+1}$

bpt $x\sqrt{12-x}+(11-x)\sqrt{x+1}-25=\frac{1}{2}(t-5)(t^2+5t+10)\geq 0 \Rightarrow t\geq 5 \Leftrightarrow 3\leq x\leq 8$

Edited by Basara, 11-10-2016 - 19:48.