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T

transformers123

$x(x^2-3)-18=0$
$\iff x^3-3x-18=0$
$\iff x^3-3x^2+3x^2-9x+6x-18=0$
$\iff x^2(x-3)+3x(x-3)+6(x-3)=0$
$\iff (x-3)(x^2+3x+6)=0$
$\iff x-3=0$ hoặc $x^2+3x+6=0$
$\iff x=3$ hoặc $(x+\dfrac{3}{2})^2+\dfrac{15}{4}=0$ (vô lí)$
Vậy ...
 
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