$A \mathbf{X}$: Product of $A$ and $ \mathbf{X}$ $A$ is $m \times n$, with columnns $\mathbf{a}_1, \mathbf{a}_2, \cdots, \mathbf{a}_n$$$\mathbf{X} \; in \; \mathbb{R}^n$$ $$A\mathbf{X}=[\mathbf{a}_1, \mathbf{a}_2, \cdots, \mathbf{a}_n]\begin{bmatrix}x_1 \\ \vdots\\ x_n \end{bmatrix}=x_1\mathbf{a}_1+x_2\mathbf{a}_2+ \cdots + x_n\mathbf{a}_n$$ → the linear combination of the columns of $A$ using t..
