21. Combinatorics and Probability

Evaluate the expression.

$\frac{12}{4!}$

Evaluate the expression.

$\frac{9!}{7!}$

Evaluate the expression.

$\frac{16!}{12!\cdot4!}$

Write the first 4 terms of the sequence $a_{n}=n^2\cdot\left(n-1\right)!$