Chapter 2 - Matrices and Systems of Linear Equations - 2.1 Matrices: Definitions and Notation - Problems - Page 122: 24

$\begin{bmatrix} 1 & 0 & 0\\ 0 & -2 & 0\\ 0 & 0 & 10\\ \end{bmatrix}$

Make $a_{ij}=0$, if $i\gt j$. That is, $a_{21}=a_{31}=a_{32}=0$ Also, $a_{ji}=a_{ij}$. So: $a_{12}=a_{21}=0$ $a_{13}=a_{31}=0$ $a_{23}=a_{32}=0$ The elements, $a_{ii}$, of the main diagonal can assume any value. Example: $\begin{bmatrix} 1 & 0 & 0\\ 0 & -2 & 0\\ 0 & 0 & 10\\ \end{bmatrix}$