In the song The Twelve Days of Christmas, my true love gave me 1 gift on the

Question:

In the song The Twelve Days of Christmas, my true love gave me 1 gift on the first day, 1 + 2 gifts on the second day, 1 + 2 + 3 gifts on the third day, and so on for 12 days.
(a) Find the total number of gifts given in 12 days.
(b) Find a simple formula for Tn, the total number of gifts given during a Christmas of n days.