(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

How is possible this equality:

[tex] {n \choose k} = \frac{n \cdot (n-1) \cdots (n-k+1)}{k(k-1)...1} = \frac{n!}{k!(n-k)!}[/tex]

? I mean where the second part [tex]\frac{n!}{k!(n-k)!}[/tex] comes from?

2. Relevant equations

3. The attempt at a solution

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# Binomial coefficient

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