Here's how it works....
2 to 3 is 1 in between. (2+1 is 3)
3 to 5 is 2 in between. (3 +2 is 5).
5 to 9 is 4 in between. (5 +4 is 9).
9 to 17 is 8 in between. (9 + 8 is 17).
So look at the numbers you are adding....1, 2, 4, 8...you are doubling that. So...
You double 8 and that is 16. So you add 16 to 17 which gives you 33. Then you do it again....16 times 2 is 32, and add that to 33.
Volia!!