How to do Factorial on TI-84:

If you’re learning math or preparing for exams, you’ll probably need factorials. Your TI-84 Plus calculator can handle factorials in just a few steps. In this guide, we’ll show you two ways to do factorial on the TI-84 – using the built-in factorial button and with a custom program.

We’ll also cover some common errors, limits of factorial calculations, and answer FAQs that students often ask.

Before diving into factorials, try them yourself on our fully functional TI-84 calculator online. This web-based emulator mimics the exact features of the TI-84 so you can follow each step without needing a physical calculator.

1. Using the built-in Factorial Function

1. Enter the number (e.g., 5)
   
2. Press the MATH key
   
3. Use the right arrow ► to go to the PROB (Probability) menu
   
4. Scroll down to option 4: ! and press ENTER
   
5. The screen will show: 5!
   
6. Press ENTER again to get the result → 120

This method can calculate up to 69! For greater numbers, the calculator will show an overflow error.

2. Writing a Custom Program (For Repeated Use)

This method is useful if you want to calculate factorials often or for multiple numbers quickly.

1. Press PRGM → right arrow to NEW → select 1: Create New
   
2. Name your program (e.g., FACT) and press ENTER
   

Enter the following code:

:Prompt N
:1→A
:For(I,1,N)
:A×I→A
:End
:Disp A

3. Press 2nd then MODE to quit and return to the home screen
   
4. Run your program:
   
   • Press PRGM → select your “program name” → press ENTER → then press ENTER again
     
5. Enter a number (e.g., 6) → factorial will display (720)

What is a Factorial?

A factorial (n!) means multiplying a number by every positive integer below it. For example:

• 5! = 5 × 4 × 3 × 2 × 1 = 120
• 0! is defined as 1
• Factorials grow extremely fast, which is why calculators like the TI-84 Plus have a limit.

Common Errors with Factorials on TI-84

• Overflow Error: Happens if you try to compute numbers above 69!.
• Negative Factorial: The TI-84 will give a domain error since factorials are only defined for non-negative integers.
• Decimal Input: Factorials are not valid for decimals. Use the Gamma function in advanced math instead.

FAQs

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