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### What Comes Ninth?

```Date: 11/07/2010 at 12:17:21
From: MONA
Subject: Sequencing numbers

3, 9, 4, 5, 25, 20, 21, 441.

What is the next number?

```

```
Date: 11/07/2010 at 12:28:42
From: Doctor Ali
Subject: Re: Sequencing numbers

Hi Mona!

Thanks for writing to Dr. Math.

See below:

3     -> raise to the second power ->   9
9     -> subtract by 5 ->               4
4     -> add 1 ->                       5

Then those three steps start again, beginning from the result of the third
operation:

5     -> raise to the second power ->   25
25    -> subtract by 5 ->               20
20    -> add 1 ->                       21

And the pattern repeats:

21    -> raise to the second power ->   441
441   -> subtract by 5 ->               ...

Does that make sense?

Please write back if you still have any difficulties.

- Doctor Ali, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Sequences, Series

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