the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n.
1) 3! = 3x2x1=6
2) 4!=4x3x2x1 =24
3) 5! =5x4x3x2x1=120
4) 2! = 2x1 =2
5) 1! = 1
6) 0! = 1
