How to Define a Matrix on the TI83 Plus
You can define up to ten matrices on the TI83 Plus graphing calculator. Each matrix can have dimensions of up to 99 X 99 and can contain only realnumber entries.
To define a matrix, follow these steps:

Press
to enter the Matrix editor menu. On the TI83, press
The dimensions appearing to the right of the first two matrices indicate that these two matrices have already been defined and are stored in the memory of the calculator.

Key in the number (1 through 9, or 0) of the matrix you want to define or redefine.
If you select an alreadydefined matrix, the following steps redefine that matrix by overwriting any existing entries. The redefined matrix replaces the original matrix in the memory of the calculator.
If all ten matrices in the Matrix editor are defined and you don’t want to sacrifice any of them in order to define a new matrix, consider saving some of the alreadydefined matrices on your PC.

Enter the dimensions of the matrix. Press [ENTER] after each entry.
The dimensions, r X c, of a matrix indicate the number of rows and columns in the matrix. The number of rows is entered first.

Enter the elements in the matrix. Press [ENTER] after each entry.
The calculator enters elements one row at a time. When you press [ENTER] after entering the last element in the first row, the calculator moves to the beginning of the second row and waits for you to make another entry.
Before accepting an entry, the calculator displays the row and column of the entry at the bottom of the screen. It also displays the value assigned to that entry. To change that value, just key in the new value and press [ENTER]. The new value can also be entered as an arithmetic expression such as 9^{2} + 7. To leave that value as it is, just press [ENTER].

When you’re finished defining your matrix, press [2nd][MODE] to exit the Matrix editor and return to the Home screen.