9! = 9 * 8 * 7 * ... * 2 * 1 = 9 * (8 * 7 * ... * 2 * 1) = 9 * 8!
8! = 8 * 7 * ... * 2 * 1 = 8 * (7 * ... * 2 * 1) = 7 * 8!
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.
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2! = 2 * 1 = 2 * (1) = 2 * 1!
Hence, we can see that n! = n * (n-1)!
So, using n = 1, we can see that 1! = 1 * 0!, which simplifies to 1 = 0!
