[Toán 9] Bài về BĐT

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lp_qt

$x^2 + y^2 + z^2 - yz - 4x - 3y \ge -7$

$ \Longleftrightarrow x^2 + y^2 + z^2 - yz - 4x - 3y +7 \ge 0$

$\Longleftrightarrow (x^2-4x+4)+\left ( z^2-yz+\dfrac{y^2}{4} \right )+\dfrac{3}{4}(y^2-4y+4) \ge 0$

$\Longleftrightarrow (x-2)^2 + (z-\dfrac{y}{2})^2+\dfrac{3}{4}(y-2)^2 \ge 0 (lđ)$
 
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