What comes next in the sequence: 1, 9, 18, 28…?

Answer 1

The differences in the terms start from 8 and increases by 1 every turn. ie.,
1 + 8 = 9
9 + 9 = 18
18 + 10 = 28
28 + 11 = 39.

Answer 2

39
It increase by 8,9,10,11.....