## Precalculus (6th Edition) Blitzer

The expression $n!$ can be calculated as $n!=\left( n \right)\cdot \left( n-1 \right)\cdot \left( n-2 \right)\cdot ...\cdot 3\cdot 2\cdot 1$.
According to the definition of the factorial of any positive number, $n!=\left( n \right)\cdot \left( n-1 \right)\cdot \left( n-2 \right)\cdot ...\cdot 3\cdot 2\cdot 1$ That means we get the product of the numbers from 1 to $n$. Point to be noted $0!=1$.