n:1,2,3,4,5,6

Question

f(n):0,1,3,6,10,15
find the nth term
and the 20th term in the sequence

Answer 1

for every letter n f(n) is added by the next number.
0,1
1,2
3,3
4,6,
5,10
6,15
7,21
8,28
9,34
10,43
11,53
12,64
13,76
14,89
15,103
16,118
17,134
18,151
19,169
20,188= answer