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Factorial of n (written n!) is the product of all positive integers from 1 to n: n! = n × (n−1) × ... × 2 × 1. Factorials appear in combinations, permutations, probability, and Taylor series expansions.

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1. 10! = 1 (by definition)
2. 21! = 1, 2! = 2, 3! = 6, 4! = 24, 5! = 120
3. 3n! = n × (n-1)!
4. 4C(n,k) = n! / (k! × (n-k)!) — factorials underlie combination formulas

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