Find
- P(3,2)
\[3 \cdot 2 = 6\]
- P(5,3)
\[5 \cdot 4 \cdot 3 = 60\]
- P(8,5)
\[8 \cdot 7 \cdot 6 \cdot 5 \cdot 4 = 6720\]
- P(1,3)
This example clearly illustrates why the formula \(\frac{n!}{(n-r)!}\) also works when \(n < r\) (you have to, at some point, multiply by 0 and that makes the whole product 0)
\[1 \cdot 0 \cdot -1 = 0\]