[Toán 9] Tìm max,min của x

V

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[TEX]\Rightarrow \left{\begin{a+b+c = 7- x}\\{a^2 + b^2 + c^2 = 13 - x^2 }[/TEX]

[TEX]A/ dung BDT: 3(a^2 + b^2 + c^2) \geq (a+b+c)^2[/TEX]

[TEX]\Rightarrow 3(13 - x^2) \geq (7- x)^2 \Leftrightarrow 2x^2 - 7x + 5 \leq 0[/TEX]

[TEX]\Leftrightarrow 1 \leq x \leq \frac{5}{2}[/TEX]
 
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