[TEX]\sqrt{\frac{x^3}{x^3 + 8y^3}} \geq \frac{x^2}{x^2 + 2y^2}(1)[/TEX]
[TEX]\Leftrightarrow \frac{x^3}{x^3 + 8y^3} \geq \frac{x^4}{(x^2 + 2y^2)^2}[/TEX]
[TEX]\Leftrightarrow (x^2 + 2y^2)^2 \geq x(x^3 + 8y^3) \Leftrightarrow 4x^2y^2 + 4y^4 \geq 8xy^3 \Leftrightarrow x^2 + y^2 \geq 2xy (dung)[/TEX]
[TEX]\sqrt{\frac{y^3}{y^3 + (x+y)^3}} \geq \frac{y^2 }{x^2 + 2y^2} (2)[/TEX]
[TEX]\Leftrightarrow \frac{y^3}{y^3 + (x+y)^3} \geq \frac{y^4 }{(x^2 + 2y^2)^2}[/TEX]
[TEX]\Leftrightarrow (x^2 + 2y^2)^2 \geq y\big( y^3 + (x+y)^3\big) \Leftrightarrow (x^2 + 2y^2)^2 - y^4 \geq y(x+y)^3 \Leftrightarrow (x^2 + y^2)(x^2 + 3y^2) \geq y(x+y)^3[/TEX]
[TEX]x^2 + y^2 \geq \frac{(x+y)^2}{2}[/TEX]
[TEX]x^2 + 3y^2 = x^2 + y^2 + 2y^2 \geq 2xy + 2y^2 = 2y(x+y)[/TEX]
[TEX]\Rightarrow (x^2 + y^2)(x^2 + 3y^2) \geq y(x+y)^3 \Rightarrow (2) dung[/TEX]
[TEX]Tu (1) , (2) \Rightarrow A \geq 1 \Leftrightarrow x=y[/TEX]