What is the nth term of 1,3,6,10,15,21

1,3,6,10,15,21, ...

These are all the sum of natural numbers upto the n

So, in this series

1st Term = 1

2nd Term = 1+2 = 3

3rd Term = 1+2+3 = 6

4th Term = 1+2+3+4 = 10

So,

Term - n = 1+2+3+ ...+n = n (n+1) / 2

That's your answer n (n+1) / 2

Good Luck