[Toán 8]Tìm GTNN.

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vietanhqlqx

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noinhobinhyen

Đặt $A = x^2+x+y^2-y+xy$

$\Rightarrow 2A=2x^2+2x+2y^2-2y+2xy$

$\Leftrightarrow 2A=(x+1)^2+(y-1)^2+(x+y)^2-2 \geq -2$

$\Rightarrow A \geq -1$

$minA = -1 \Leftrightarrow x=-1 ; y=1$
 
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