INTRODUCTION TO MATRICES
• PRACTICE (online exercises and printable worksheets)

A matrix (pronounced MAY-trix) is a rectangular arrangement of numbers, like: $$\begin{bmatrix} 7 & 3 & 2\\ 6 & 5 & 8 \end{bmatrix}$$ A number in a matrix is often called an element, a member, or an entry of the matrix.
The members of a matrix are often enclosed in (square) brackets.

Rows have a horizontal orientation, and are numbered from top to bottom:

 $7$ $3$ $2$ $6$ $5$ $8$

Thus,   $\begin{bmatrix} 7 & 3 & 2\\ \end{bmatrix}$   is the first row, and   $\begin{bmatrix} 6 & 5 & 8\\ \end{bmatrix}$   is the second row.

Columns have a vertical orientation, and are numbered from left to right:

 $7$ $3$ $2$ $6$ $5$ $8$

Thus,   $\begin{bmatrix} 7\\ 6 \end{bmatrix}$   is the first column,   $\begin{bmatrix} 3\\ 5 \end{bmatrix}$   is the second column, and   $\begin{bmatrix} 2\\ 8 \end{bmatrix}$   is the third column.

The plural of matrix is matrices (pronounced MAY-tri-sees).

Matrices offer a way to represent large amounts of data in an organized way.
Matrices are particularly well-suited to computer analysis.
There are a multitude of applications of matrices, including:

• encrypting numerical data (think national security)
• computer graphics (think video games)
• solving systems of equations

The size of a matrix is reported by stating the number of rows, followed by the ‘$\,\times\,$’ symbol, followed by the number of columns.
For example, the size of

$\begin{bmatrix} 7 & 3 & 2\\ 6 & 5 & 8 \end{bmatrix}$

is $\,2\times 3\,$, which is read aloud as ‘$\,2\,$ by $\,3\,$’.

Observe that an $\,m \times n\,$ matrix has $\,mn\,$ elements,
since there are $\,m\,$ rows, with $\,n\,$ entries in each row.

Matrices are usually named with capital letters.
Members of a matrix are usually named with lowercase letters.
In particular, the elements of a matrix $\,M\,$ are conventionally named $\,m_{ij}\,$:

• $\,m_{ij}\,$ is read aloud as ‘$\,m\,$ sub $\,i\,$ $\,j\,$’
• the $\,i\,$ and $\,j\,$ are called subscripts; they are written a little bit below the line
• the first subscript ($\,i\,$) gives the row number of the element
• the second subscript ($\,j\,$) gives the column number of the element
• a capital letter is used to denote the matrix, and the corresponding lowercase letter is used to denote the elements

For example, if

$M = \begin{bmatrix} 7 & 3 & 2\\ 6 & 5 & 8\end{bmatrix}$

then:
$\,m_{11} = 7\,$   (first row, first column; read as em sub one one, NOT (say) em sub eleven)
$\,m_{12} = 3\,$   (first row, second column)
$\,m_{13} = 2\,$   (first row, third column)
$\,m_{21} = 6\,$   (second row, first column)
$\,m_{22} = 5\,$   (second row, second column)
$\,m_{23} = 8\,$   (second row, third column)

Two matrices are equal when they have the same size, and corresponding elements are equal.
Precisely, we have:

EQUALITY OF MATRICES
Let $\,A\,$ and $\,B\,$ be matrices.
Then,
 $A=B$ if and only if $A\,$ and $\,B\,$ have the same size and $\,a_{ij} = b_{ij}\,$ for all $\,i\,$ and $\,j\,$.
Here, $\,i\,$ takes on all possible row numbers, and $\,j\,$ takes on all possible column numbers.

A matrix with the same number of rows and columns is called a square matrix.
For example, $\begin{bmatrix} 1 & 2\\ 3 & 4 \end{bmatrix}$ is a square matrix.

A matrix where all the entries are zero is called a zero matrix.
For example, $\begin{bmatrix} 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 \end{bmatrix}$ is a $\,2\times 4\,$ zero matrix.

Master the ideas from this section