Determine a formula involving binomial coefficients for the nth term of a sequence if its initial terms are those listed. [Hint: Looking at Pascal's triangle will be helpful. Although infinitely many sequences start with a specified set of terms, each of the following lists is the start of a sequence of ... $1, 3, 15, 84, 495, 3003, 18564, 116280, 735471, 4686825,\dots$

Give a combinatorial proof that if n is a positive integer then $\displaystyle\sum_{k = 0}^{n} k^{2} \binom{n} {k} = n(n + 1)2^{n−2}.$ [Hint: Show that both sides count the ways to select a subset of a set of $n$ elements together with two not necessarily distinct elements from this subset. Furthermore, express the right-hand side as $n(n − 1)2^{n−2} + n2^{n−1}.]$

Use question $33$ to prove the hockeystick identity from question $27.$ [Hint: First, note that the number of paths from $(0, 0)\: \text{to}\: (n + 1,r)$ equals $\binom{n + 1 + r}{r}.$ Second, count the number of paths by summing the number of these paths that start by going $k$ units upward for $k = 0, 1, 2,\dots,r.]$

Use question $33$ to prove Pascal’s identity. [Hint: Show that a path of the type described in question $33$ from $(0, 0)\: \text{to}\: (n + 1 − k, k)$ passes through either $(n + 1 − k, k − 1)\: \text{or} \:(n − k, k),$ but not through both.]