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

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. 

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Vandana D. | Tutor for elementary and maths (up to 5 grade)Tutor for elementary and maths (up to 5...
-1
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
 
Ryan S. | Mathematics and StatisticsMathematics and Statistics
4.8 4.8 (10 lesson ratings) (10)
-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

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However, 7*3^2=63 , not 72. A possibility for the even-numbered terms is 3*7, (3^2)*8, (3^3)*9=243,...
Ralph L. | Algebra I, II, Visual Basic, Beginning C++ tutorAlgebra I, II, Visual Basic, Beginning C...
4.0 4.0 (1 lesson ratings) (1)
-1
I think it's 243.
 
7  (7*3) 8 (8*9) 9 (9*27)  --->7 21 8 72 9 243
Michael F. | Mathematics TutorMathematics Tutor
4.6 4.6 (5 lesson ratings) (5)
-3
7, 7×3, 8, 8×32, 9, 9×33,....n, followed by n×3n-6  for example