[tex]\left\{\begin{matrix} f(x)+3f(\frac{1}{x})=x & \\ f(\frac{1}{x})+3f(x)=\frac{1}{x} & \end{matrix}\right.\\ \\ \Leftrightarrow \left\{\begin{matrix} f(x)+3f(\frac{1}{x})=x & \\ 3f(\frac{1}{x})+9f(x)=\frac{3}{x} & \end{matrix}\right.\\ \\ \Rightarrow 8f(x)=\frac{3}{x}-x\Leftrightarrow f(x)=\frac{3}{8x}-\frac{x}{8}[/tex]