5. (20%) In the following, you should use uppercase letters to denote matrices and lowercase letters with subscripts to denote their entries. For example, if M is a matrix, then $m_{ij}$ denotes its (i,j)-entry. Use lowercase letters with an arrow to denote column vectors of a matrix. For example, $\vec{m}_i$ denotes the i-th column vector of the matrix
M.
In your proofs, use the following definitions as needed.
Definition (Equality of matrices): Matrices A and B are equal if they have the same size and have equal corresponding entries $a_{ij} = b_{ij}$ for all (i,j).
Definition (Matrix transpose): The transpose of an $m \times n$ matrix A is the $n \times m$ matrix denoted by $A^T$ whose (i,j)-entry is the (j,i)-entry of A.
Definition (Linear combination): A linear combination of vectors $\vec{u}_1, \vec{u}_2, ..., \vec{u}_k$ is a vector of the form $c_1\vec{u}_1 + c_2\vec{u}_2 + \cdots + c_k\vec{u}_k$, where $c_1, c_2, ..., c_k$ are scalars.
Definition (Standard vectors): $$\vec{e}_1 = \begin{bmatrix} 1 \\ 0 \\ \vdots \\ 0 \end{bmatrix}, \vec{e}_2 = \begin{bmatrix} 0 \\ 1 \\ \vdots \\ 0 \end{bmatrix}, ..., \vec{e}_n = \begin{bmatrix} 0 \\ 0 \\ \vdots \\ 1 \end{bmatrix}$$
Now let A denote a $m \times n$ matrix. Prove that $(A^T)^T = A$ and $A\vec{e}_i = \vec{a}_i$ for $i = 1, ..., n.
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