Suppose $A$ be an $m \times n$ and $B$ be $m \times p$. Prove that
$$R(A \mid B)=R(A) + R(B)$$
where $R(A)=\{Ax \mid x \in \mathbb{R}^n\}$ is the range of $A$.
Meyer's book (Matrix analysis and applied linear algebra) question 4.2.9.
I cannot understand what $A \mid B$ is so I cannot prove this problem.