Answer:
See below in bold.
Step-by-step explanation:
The formula is Tn = n(n + 1) / 2
The first triangular number is 1(1+1) / 2 = 2/2 = 1.
So the 15th = 15(15+1) / 2 = 120.
and the 19th = (19 * 20) / 2 = 190.
