T-Rex: On the n+1th day of Christmas
T-Rex: MY TRUE LOVE SENT TO ME:
T-Rex: Everything from days 1 through to n, plus n+1 additional items. On day n I had n(n+1)(n+2)/6 items.
T-Rex: Adding [the sum from k=1 to n of k = n(n+1)/2] gives (n+1)(n+2)(n+3)/6
T-Rex: Therefore by induction on day n+1 I have the (n+1)th tetrahedral number
T-Rex: of useless gifts that I can't return!
The author writes:
Dear True Love,
Thank you for the lovely gifts. Where the heck do you buy swans, anyway?