Solution 14286: Calculating Factorials (!) Using the TI-83 Family, TI-84 Plus Family, or TI-Nspire™ Handheld in TI-84 Plus Mode.
How do I calculate a factorial using the TI-83 family, TI-84 Plus family, or TI-Nspire™ handheld in TI-84 Plus mode?
The instructions below display how to calculate a factorial.
Example: Find 6!.
1) On the home screen, press [CLEAR] to clear any previous entries.
2) Press  [MATH] and the left arrow key to select the PRB menu.
3) Press  for !.
4) Press [ENTER] to display the answer of 720.
Please Note: The factorial is computed recursively using the relationship (n+1)! = n*n! until n is reduced to either 0 or -1/2. At that point, the definition 0!=1 or the definition (-1/2)!= Öp is used to complete the calculation.
! (factorial) returns the factorial of either an integer or a multiple of 0.5. For a list, it returns factorials for each integer or multiple of 0.5. The factorial value must be greater than or equal to -0.5 and less than or equal to 69.
Please see the TI-83 family and TI-84 Plus family guidebooks for additional information.
TI-Nspire users may refer to the TI-84 Plus family guidebook.