[Toán 9]

E

eye_smile

Rg đc:
$M=\dfrac{\sqrt{x-1}}{x}+\dfrac{\sqrt{y-2}}{y}+\dfrac{\sqrt{z-3}}{z}$

Có: $x-1+1 \ge 2\sqrt{x-1}$
\Rightarrow $\dfrac{\sqrt{x-1}}{x} \le \dfrac{1}{2}$
TT có: $\dfrac{\sqrt{y-2}}{y} \le \dfrac{1}{2\sqrt{2}}$
$\dfrac{\sqrt{z-3}}{z} \le \dfrac{1}{2\sqrt{3}}$

\Rightarrow $M \le....$
Dấu "=" xảy ra \Leftrightarrow $x=2;y=4;z=6$
 
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