\[\boxed{\text{839\ (839).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \frac{(n + 1)!}{n!} = \frac{n! \cdot (n + 1)}{n!} =\]
\[= n + 1.\]
\[\textbf{б)}\ \frac{n!}{(n + 2)!} =\]
\[= \frac{n!}{n!(n + 1)(n + 2)} =\]
\[= \frac{1}{(n + 1)(n + 2)}\]
\[\textbf{в)}\ \frac{(n + 3)!}{(n + 1)!} =\]
\[= \frac{(n + 1)!(n + 2)(n + 3)}{(n + 1)!} =\]
\[= (n + 2)(n + 3).\]
\[\textbf{г)}\ \frac{(n + 1)!(n + 3)}{(n + 4)!} =\]
\[= \frac{(n + 1)(n + 3)}{(n + 1)!(n + 2)(n + 3)(n + 4)} =\]
\[= \frac{1}{(n + 2)(n + 4)}.\]
\[\textbf{д)}\ \frac{(n + 11)!n}{(n + 10)!} =\]
\[= \frac{(n + 10)!(n + 11) \cdot n}{(n + 10)!} =\]
\[= (n + 11) \cdot n = n² + 11n.\]