15. Sequences, Series, and the Binomial Theorem

Write the first 4 terms of the sequence $a_n=n^2\cdot (n-1)!$

Evaluate the expression.

$\frac{12}{4!}$

Evaluate the expression.

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

Evaluate the expression.

$\frac{16!}{12!\cdot 4!}$