Verbally describe this pattern

Question

1,3,6,10,15,21

Answer 1

The nth term is the sum of the first n positive integers.

Or, starting with 1, add 1. Then increase the amount added by 1 for each term. That is, the next amounts added are
2,3,4,5,...

Answer 2

add the last two numbers in the begining add 1 three times