$$ \left\{ \begin{array}{ll} \vec{x}, \vec{y} \in \mathbb{R}^n, & f(\vec{x}+\vec{y})=f(\vec{x})+f(\vec{y}) \\ 4 -5 & 600 \\ \end{array} \right. \Leftrightarrow $$

$$ \vec{x}\in R\mathbb{R}\left\{ \begin{array}{rrr} -1 & 20 & f(\vec{x})=f(\vec{x}) \\ 4 & -5 & 600 \\ \end{array} \right. \Leftrightarrow $$