Factorials and arrangements

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Learn it $n! = n \times (n - 1) \times \dots \times 1$ counts the ways to arrange n different things. $0! = 1$. In fractions of factorials, cancel the common part.

Example: Simplify $\frac{10!}{8!}$.

$10! = 10 \times 9 \times 8!$.
Cancel 8!: $10 \times 9$.
$= 90$.

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