1. What's the rule of 5, 17, 49? It's about sequence

1. What's the rule of 5, 17, 49? It's about sequence

1=（1-1）×2+1
5=（2-1）×2×2+1
17=（3-1）×2×2×2+1
49=（4-1）×2×2×2×2+1

Division addition sequence: 5,17,21,25, () A.30 b.31c.32d.34

B
5,17,21,25,( 31)
1.5, 17, 21 and 2 are all odd numbers, while only 31 in the options are odd numbers
2. Single digit plus ten digit 5 8 3 7 4 5 + 3 = 8 3 + 4 = 7, that is odd item plus ten digit = even item plus ten digit

2,5,10,17,26,37,50,65, ···············

It's not an equal ratio sequence
The difference was 3.5.7.9.11.13
Suppose the sequence is {an}
So an-a (n-1) is an arithmetic sequence
Will that be true