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E

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+$y=0$ \Rightarrow $A=1$

+$y$ khác 0

$A=\dfrac{(\dfrac{x}{y})^2-\dfrac{x}{y}+1}{(\dfrac{x}{y})^2+\dfrac{x}{y}+1}$

Đặt $a=\dfrac{x}{y}$

\Rightarrow $A=\dfrac{a^2-a+1}{a^2+a+1}$

\Leftrightarrow $a^2(A-1)+a(A+1)+A-1=0$

%%- $A=1$

%%- $A$ khác 1

$\Delta=(A+1)^2-4(A-1)^2=-3A^2+10A-3 \ge 0$

\Leftrightarrow $\dfrac{1}{3} \le A \le 3$
 
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