(Toán 9) CMR:

E

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$\dfrac{1}{xy}+\dfrac{1}{xz} \ge \dfrac{4}{xy+xz}=\dfrac{4}{x(y+z)}$

Có: $4=x+(y+z) \ge 2\sqrt{x(y+z)}$

\Rightarrow $x(y+z) \le 4$

\Rightarrow $VT \ge 1$

Dấu "=" xảy ra \Leftrightarrow $x=2;y=z=1$
 
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