How to Store Variables on the TI83 Plus
If you plan to use the same number many times when evaluating arithmetic expressions with the TI83 Plus, consider storing that number in a variable. To do so, follow these steps:

If necessary, press [2nd][MODE] to enter the Home screen.

Enter the number you want to store in a variable.
You can store the number as an arithmetic expression. This is illustrated in the first picture. After you complete the steps for storing the number, the calculator evaluates that expression.

Press
The result of this action is shown in the first picture.

Press [ALPHA] and then press the key corresponding to the letter of the variable in which you want to store the number.
The second picture shows this process. The letters used for storing variables are the letters of the alphabet and the Greek letter

Press [ENTER] to store the value.
This is illustrated in the third picture.
The number you store in a variable remains stored in that variable until you or the calculator stores a new number in that variable. Because the calculator uses the letters X, T, and
when graphing functions, parametric equations, and polar equations, it is possible that the calculator will change the value stored in these variables when the calculator is in graphing mode. For example, if you store a number in the variable X and then ask the calculator to find the zero of the graphed function X^{2}, the calculator will replace the number stored in X with 0, the zero of X^{2}.
So avoid storing values in these three variables if you want that value to remain stored in that variable after you have graphed functions, parametric equations, or polar equations.
After you have stored a number in a variable, you can insert that number into an arithmetic expression. To do so, place the cursor where you want the number to appear, press [ALPHA], and then press the key corresponding to the letter of the variable in which the number is stored.