0.4 Matrix concepts -- matrices on calculators

Many modern graphing calculators have all the basic matrix operations built into them. The following is a brief overview of how to work with matrices on a TI-83, TI-83 Plus, TI-84, or TI-84 Plus.

The calculator has room to store up to ten matrices at once. It refers to these matrices as [A], [B], and so on, through [J]. Note that these are not the same as the 26 lettered memories used for numbers.

The following steps will walk you through the process of entering and manipulating matrices.

• Hit the MATRX button. On a TI-83, this is a standalone button; on a TI-83 Plus, you first hit 2nd and then MATRIX (above the $x^{\mathrm{–1}}$ button). The resulting display is a list of all the available matrices. (You have to scroll down if you want to see the ones below [G] .)

• Hit the right arrow key ► twice, to move the focus from NAMES to EDIT . This signals that you want to create, or change, a matrix.

• Hit the number 1 to indicate that you want to edit the first matrix, [A] .
• Hit 4 ENTER 3 ENTER to indicate that you want to create a 4x3 matrix. (4 rows, 3 columns.)

• Hit 1 ENTER 2 ENTER 3 ENTER 4 ENTER 5 ENTER 6 ENTER7 ENTER 8 ENTER 9 ENTER 10 ENTER 11 ENTER 12 ENTER This fills in the matrix with those numbers (you can watch it fill as you go). If you make a mistake, you can use the arrow keys to move around in the matrix until the screen looks like the picture below.

• Hit 2nd Quit to return to the main screen.
• Return to the main matrix menu, as before. However, this time, do not hit the right arrow to go to the EDIT menu. Instead, from the NAMES menu, hit the number 1 . This puts [A] on the main screen. Then hit ENTER to display matrix [A] .

• Go through the process (steps 1-7) again, with a few changes. This time, define matrix [B] instead of matrix [A] . (This will change step 3: once you are in the EDIT menu, you will hit a 2 instead of a 1.) Define [B] as a 3x2 matrix in step 4. Then, in step 5, enter the following numbers:
  $\left[\begin{array}{cc}\text{10}& \text{40}\\ \text{20}& \text{50}\\ \text{30}& \text{60}\end{array}\right]$
  When you are done, and have returned to the main screen and punched 2 in the NAMES menu (step 7), your main screen should look like this:

• Now, type the following keys, watching the calculator as you do so. TI-83 Plus users should always remember to hit 2nd MATRIX instead of just MATRX. MATRX 1 + MATRX 2
  This instructs the computer to add the two matrices. Now hit ENTER
  Hey, what happened? You asked the computer to add two matrices. But these matrices have different dimensions . Remember that you can only add two matrices if they have the same dimensions—that is, the same number of rows as columns. So you got an “Error: Dimension Mismatch.” Hit ENTER to get out of this error and return to the main screen.
• Now try the same sequence without the + key: MATRX 1 MATRX 2 ENTER
  This instructs the calculator to multiply the two matrices. This is a legal multiplication—in fact, you may recognize it as the multiplication that we did earlier. The calculator displays the result that we found by hand: $\left[\begin{array}{ccc}1& 2& 3\\ 4& 5& 6\\ 7& 8& 9\\ \text{10}& \text{11}& \text{12}\end{array}\right]$ $\left[\begin{array}{cc}\text{10}& \text{40}\\ \text{20}& \text{50}\\ \text{30}& \text{60}\end{array}\right]$ = $\left[\begin{array}{cc}\text{140}& \text{320}\\ \text{320}& \text{770}\\ \text{500}& \text{1220}\\ \text{680}& \text{1670}\end{array}\right]$
• Enter a third matrix, matrix [C]= $\left[\begin{array}{cc}3& 4\\ 5& 6\end{array}\right]$ . When you confirm that it is entered correctly, the screen should look like this:
  Now type MATRX 3 x-1 ENTER
  This takes the inverse of matrix [C]. Note that the answer matches the inverse matrix that we found before.
• Type MATRX 3 x-1 MATRX 3 ENTER
  This instructs the calculator to multiply matrix [C]-1 times matrix [C]. The answer, of course, is the 2×2 identity matrix [I].