Introduction: Half-Life Calculator Program (Ti-89, Ti-84, and Ti-83)

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The purpose of this Instructable is to show you how to write a half-life program on your calculator. The program will make it very quick and easy to find to half-life, initialmass, time, or final mass. All you have to do is enter three of the variables (I will explain more on this later) and the calculator will compute the fourth one. This means no more logarithms or decimal exponents!

Step 1: Creating the Program

 The name of the program can be whatever you want.  I simply called it "hl" because it is nice and short.

Ti-89
To create the program, just follow the pictures. 

Ti-83 & Ti-84
I can't take screen shots on my Ti-83 so you will just have to follow my directions.
1. Press PRGM and arrow over to "NEW" and
2. Hit enter and give the program a name. (It must be 8 or less characters)

Step 2: Writing the Program

The easiest way to write the program is to look at the extended screenshot I made.  It is crucial to type everything EXACTLY as it is in the picture.  One error could mess up the entire program. Make sure you type the correct amount of parentheses.

Ti-89
Depending on how familiar you are with programing, you might not know how to enter some of the commands.  Here's how to type all the symbols you will need.

The colon represents one line of commands.  Hit enter to get to the next line. To type the extra colons on line four, press 2nd then 4.
To input "Prompt" press F3, 5
The fastest way to type letters is by pressing "alpha" twice to turn on alpha lock
For the "If  Then" command, press F2, 2, Enter
The arrow is created by pressing the STO> button (it's right above the ON button)
To input "Disp" press F3, 2
You can type quotes by pressing 2nd, 1

Ti-83 & Ti-84
*Please read the list below because your program won't look exactly like the one in the picture!

Your program will start off differently from the picture.  Type (To type "Input" press PRGM, right arrow, enter)(To type the extra colons, press "alpha", ".")    Input "halflife",x:Input "mass",y:Input "time",z:Input "mass2",t
The colon represents one line of commands.  Hit enter to get to the next line.
To input "Prompt" press PRGM, right arrow, 2
The fastest way to type letters is by pressing 2nd, "alpha" to turn on alpha lock
For the "If " command, press PRGM, 1
To type the equals sign, press 2nd, "MATH", enter
For the "Then" command (This should be on its own line for the Ti-83 and 84), press PRGM, 2
Where the picture says "EndIf" type "End" (PRGM, 7) instead
The arrow is created by pressing the STO> button (it's right above the ON button)
You can type quotes by pressing "alpha", "+"

Step 3: Using the Half-Life Program

In order to use this program you must input three variables and the calculator will give you the fourth one.  The four variables are the half-life, Initial mass, time, and the second mass.  You must input 0 (zero) for the variable that you want the calculator to find.

Here's how to start the program

Ti-89
Type the name of your program in the home screen the end it with two parentheses. Then hit enterMine looks like this: Ex. hl()

Ti-83 &Ti-84
Press PRGM (as if you haven't done that enough yet!) then use the arrows or numbers to select the half-life program.  Then hit enter.

Step 4: Using the Program

I will finally show you the four ways this program can be used. In the red is the half-life of carbon-14, green is the mass, blue is the time, and the yellow is the second mass.

1. Solve for the second mass (yellow)
Type in 5730 and hit enter
Type in 128 and hit enter
Type in 11460 and hit enter
To solve for "Mass2" type in 0 (zero)
*The answer should be 32*

2. Solve for time (blue)
Type in the half-life, mass and mass2
Type zero for the time
*The answer should be 11460*

3. Solve for Mass (the first one) (green)
Type in half-life, time and mass2
Type zero in for "Mass"  
*The answer should be 128*

4. Solve for Half-life
Type in mass, time and mass2
Type in zero for "halflife"
*The answer should be 5730*

If you do not get the right answer, double-check your program for errors.  If you need more help with anything, even if it's not just about this program don't hesitate to ask!

Here are the links to my other programs (they are meant for the Ti-83 and 84 but you can pretty easily change them for the Ti-89)
https://www.instructables.com/id/Quadratic-Formula-for-the-TI-83-and-84/
https://www.instructables.com/id/Herons-Formula-program-for-the-TI-83-and-84/