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 $k = 0, 1, 2,\dots,r.]$
