0! = 1 (zero factorial equals one)
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5! = 5x4x3x2x1 = 120
4! = 4x3x2x1 = 24
3! = 3x2x1 = 6
2! = 2x1 = 2
1! = 1
0! = 1
If n! is the number of ways to put n objects in order, then 0! is the number of ways to put 0 objects in order. The only way to put 0 objects in order is to "Not have any objects." This is 1 way. Therefore 0! = 1.
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It does sound strange to say that 0! equals 1!
( this is not one factorial, just stressing the phrase :-P )
BUT, do you know why 0! = 1 ?
We say that 0! = 1 just for formulas like the binomial coefficient to work,
imagine the “n choose k” scenario, we have:
n!/k!(n-k)!
and we know when n = k this should equals 1, right ? so...
n!/n!(n-n)!
n!/n!0!
If we had that 0! doesn't equal 1, we wouldn't be able to use this formula,
so we all agree that 0! equals 1, and we are happy about that. ;)