A. By definition a factorial means multiplying every positive integer less than and including that number.
1. So n! = n x (n-1) x (n-2) x...x 3 x 2 x 1
2. Also, 0! = 1 by definition.
Note: The above steps are very important to know for problems involving probability.
Ex [1] 5! = __________.
a. 5! = 5 x 4 x 3 x 2 x 1 = 120.
b. The answer is 120.
Ex [2] 7! = __________.
a. 7! = 7 x 6 x 5 x 4 x 3 x 2 x 1 = 5040.
b. The answer is 5040.
B. It will help considerably if you can memorize the factorials at least to 7. Below is a table of the factorials to 7:
| 0! = | 1 | 1! = | 1 |
| 2! = | 2 | 3! = | 6 |
| 4! = | 24 | 5! = | 120 |
| 6! = | 720 | 7! = | 5040 |