đề thi

E

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Giải:
Đặt $x = b + c - a;y = c + a - b;z = a + b - c$
\Rightarrow $2a = y + z;2b = x + z;2c = x + y$
Ta có;
$2A = \dfrac{{2a}}{{b + c - a}} + \dfrac{{2b}}{{a + c - b}} + \dfrac{{2c}}{{a + b - c}} = \dfrac{{y + z}}{x} + \dfrac{{x + z}}{y} + \dfrac{{x + y}}{z} = \left( {\dfrac{x}{y} + \dfrac{y}{x}} \right) + \left( {\dfrac{y}{z} + \dfrac{z}{y}} \right) + \left( {\dfrac{x}{z} + \dfrac{z}{x}} \right) \ge 6$
\Rightarrow $A \ge 3$
đpcm
 
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