Can someone explain how this:
$$n!\cdot(n + 2)!$$
could become:
$ n\cdot(n + 2)!\cdot(n - 1)!$
$$ n! = n \cdot (n - 1)! $$ That's all.
You have
$$n!=1\times 2\times\cdots\times n$$
so
$$n!=(1\times\cdots \times n-1)\times n=(n-1)!\times n.$$
Therefore
$$n!\cdot (n+2)!=n\cdot (n-1)!\cdot (n+2)! = n\cdot (n+2)!\cdot (n-1)!.$$
And that's it.
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$$ n! = n \cdot (n - 1)! $$ That's all.