Row Operation to Clear First Column of Matrix/Examples/Arbitrary Matrix 2

Example of Use of Row Operation to Clear First Column of Matrix
Let $\mathbf A$ be the matrix:


 * $\mathbf A = \begin {pmatrix} 1 & 0 & 1 & 1 \\ -1 & 0 & 2 & 1 \\ -1 & 1 & 5 & 9 \end {pmatrix}$

The matrix $\mathbf R$ corresponding to the row operation to clear the first column is:


 * $\mathbf R = \begin {pmatrix} 1 & 0 & 0 \\ 1 & 1 & 0 \\ 1 & 0 & 1 \end {pmatrix}$

and the matrix $\mathbf B$ which results from the row operation $\mathbf R$ is:


 * $\mathbf B = \begin {pmatrix} 1 & 0 & 1 & 1 \\ 0 & 0 & 3 & 2 \\ 0 & 1 & 6 & 10 \end {pmatrix}$

Proof
We use Row Operation to Clear First Column of Matrix as follows:


 * $(1): \quad$ Apply the elementary row operation $r_2 \to r_2 + r_1$ to to add $1$ times row $1$ to row $2$.

From Elementary Matrix corresponding to Elementary Row Operation: Scale Row and Add, this is accomplished by pre-multiplying by the matrix:


 * $\mathbf E_1 := \begin {pmatrix} 1 & 0 & 0 \\ 1 & 1 & 0 \\ 0 & 0 & 1 \end {pmatrix}$

which converts $\mathbf A$ to $\mathbf A_1$ as follows:


 * $(2): \quad$ Apply the elementary row operation $r_3 \to r_3 + r_1$ to add $1$ times row $1$ to row $3$.

From Elementary Matrix corresponding to Elementary Row Operation: Scale Row and Add, this is accomplished by pre-multiplying by the matrix:


 * $\mathbf E_2 := \begin {pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 1 & 0 & 1 \end {pmatrix}$

which converts $\mathbf A_1$ to $\mathbf A_2$ as follows:

The matrix is in the correct form, and so:
 * $\mathbf B = \mathbf A_2$

Hence we can calculate $\mathbf R$: