Ta có: [tex]\frac{1}{x^3}+\frac{1}{y^3}=(\frac{1}{x}+\frac{1}{y})^2\Leftrightarrow (\frac{1}{x}+\frac{1}{y})(\frac{1}{x^2}-\frac{1}{xy}+\frac{1}{y^2})=(\frac{1}{x}+\frac{1}{y})^2\Leftrightarrow \frac{1}{x}+\frac{1}{y}=\frac{1}{x^2}-\frac{1}{xy}+\frac{1}{y^2}\geq \frac{1}{4}(\frac{1}{x}+\frac{1}{y})^2\Rightarrow \frac{1}{x}+\frac{1}{y}\leq 4\Rightarrow \frac{1}{x^3}+\frac{1}{y^3}=(\frac{1}{x}+\frac{1}{y})^2\leq 16[/tex]