combination:
$$
\overrightarrow{v_1}, \overrightarrow{v_2}, \overrightarrow{v_3} \cdots \overrightarrow{v_n} \in \mathbb{R}^n c_1 \overrightarrow{v_1}+c_2 \overrightarrow{v_2}+\cdots+c_n \overrightarrow{v_n}\left[\left(c_1 \rightarrow c_n\right) \in \mathbb{R}\right]
$$
span: $\vec{a}, \vec{b} \in \mathbb{R}$
(1) range of $n \vec{a}+m \vec{b} \quad(m, n \in \mathbb{R})$
$\operatorname{span}(\vec{a}, \vec{b})=\mathbb{R}^2$
(2) if $\vec{a}+\vec{b}=0$ then $\operatorname{span}(\vec{a}, \vec{b})=$ line
(3) $\operatorname{span}(\overrightarrow{0})=0$
$\operatorname{span}(\vec{a})=n \vec{a} \quad(line)$
(4)
$\operatorname{span}\left(\overrightarrow{v_1}, \overrightarrow{v_2}, \overrightarrow{v_3}, \cdots \overrightarrow{v_n}\right)$
$=c_1 \overrightarrow{v_1}+c_2 \overrightarrow{v_2}+c_3 \overrightarrow{v_3}+\cdots+c_n \overrightarrow{v_n} \mid c_i \in \mathbb{R}$ for $i \leq i \leq n$
