How to Write a Linear Interpolation Program on a TI-89

Things to know before you start.

Key titles will be in parentheses (ex. (ENTER)) and statements in quotes are exact information displayed on the screen. Important keys and text strings being introduced in each step are highlighted in the figures. When using the (ALPHA) key to get in and out of alphabet-lock, you can check which mode the calculator is in by looking for the box along the bottom of the screen marked in figure 1 by the green rectangle. If the box is present the calculator is in a-lock if there is nothing you are in standard mode.

-Turn on calculator with the (ON) button in the lower left-hand corner.

-Press (APPS) to make sure you are in the application center and scroll to find the program editor app and press (ENTER).

-Select option 3, “New…” by pressing the (3) button.

-Make sure your program has the following settings, “Type: program” and “Folder: main."

-Move your cursor to the “Variable:” section, triple tap (ALPHA) to lock the keys in alphabet mode, and type the letters to name the program “interp.” Once this is done press (ENTER) twice to move to the program editor.

-First move the cursor down to the open line. Press the (CATALOG) key, then scroll down to “NewProb” and press (ENTER) twice.

-Press (F4) at the top then press (3) to select “Local.” Next press the (ALPHA) key, then the letter (a), and then the (,) button. Repeat this same three key strokes ((ALPHA), (letter), (,) ) for the letters b, c, d, and e (there should be no comma after e). When finished press (ENTER).

-Press (F3) followed by the (3) key to select “Input.”

-You will now write the input prompt. Start by pressing (2ND) followed by the (1) key to open the prompt. Double tap the (ALPHA) key and next type, “enter the given temp” (the key marked ((-)) is the space bar) followed by the (2ND) button then the (1) button to close the prompt.

-Press (ALPHA), the (,) key, (ALPHA), (=), and then “ENTER.” You have now written the prompt for the first variable.

-You now need to define the next 4 variables in the same manor. While holding the (↑) key tap the up arrow twice followed by the green diamond and the (↑) key again to copy the prompt. Move the cursor down to the open line and tap green diamond then “ESC.” This will paste in the first prompt, so you will need to double tap (ALPHA) and change the prompt to read, “Input ’enter the upper temp’,b.” You can delete characters with the (←) key. When finished press the (ENTER) button and repeat the process of pasting, editing the prompt (double tapping the (ALPHA) key is not necessary when repeating the process) until you have a total of five prompts. The last three reading “Input ’enter the lower temp’,c”, “Input ’enter the upper entropy’,d”, and “Input ’enter the lower entropy’,e.”

-Make sure there is an open line space then press (F3) then the (2) key to start writing the display. Press the (2ND) followed by the (1) to open the display. Triple tap the (ALPHA) key and type “solution” followed by the (2ND) and (1) to close the first display.

-Press “ALPHA” then type (,) then write the equation as shown, (e+(a-c)*(d-e)/(b-c)).

-Press the (HOME) key to open the action window.

-Then press (2ND) followed by (-) to open the VAR-LINK window. Scroll down to your program titled, “interp” and press (ENTER.) Complete the program by pressing ( ) ) and then (ENTER) to start the program. To test run the program enter the values a=150, b=200, c=100, d=200, e=100 when prompted, you only need to type the numbers. The solution will be 150 if you have successful created the linear interpolation program.