What is the next number in this sequence 7 21 8 72 9

Question

What is the next number in this sequence 7 21 8 72 9.....I need to answer this question for a test and math has been a long time ago.  What is the best way to relearn math sequences and probability and statistic. 

Answer 1

One possible pattern is that the odd-numbered terms  are 7, 8, 9, 10, 11, ... and the even numbered terms are 7*3, 7*3^2, 7*3^3,...

Given this pattern the next term is 189

Answer 2

I guess the answer is 729.

 

Series is 7 21 8 72 9

showing 7*3= 21

8*3^2=8*9=72

Following this series next number could be 9*9^2=9*81=729

 

another pattern could be

7*3=21

8*3^2=72

9*3^3=243

 

Answer 3

I think it's 243.

 

7  (7*3) 8 (8*9) 9 (9*27)  --->7 21 8 72 9 243

Answer 4

7, 7×3, 8, 8×3², 9, 9×3³,....n, followed by n×3^n-6  for example