Math Is Fun Forum

yoyodragon

Guest

help on strange series!

Hi, there is this math quiz question on series:

2nd term is 9
3rd term is 28
4th term is 36
12th term is 0

Does anyone know the formula relating to this series?

It is definitely not an arithmetic nor geometric series, as far as i can tell.

Thank you for your help, i have been puzzled by this for a long time.

John E. Franklin

Member

Registered: 2005-08-29

Posts: 3,588

Re: help on strange series!

Here is a possible solution.
You start on 1 (one), and add 8, then add 19.
Add 8 and add 19 repeatedly.
When you get to 72 or 144 (multiples of 72 or 144, then you go back to zero)
With these rules,
you get this series:
1 9 28 36 55 63 10 18 37 45 64 0

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igloo myrtilles fourmis

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: help on strange series!

As John has demonstrated, there are many possible solutions because you can engineer as many patterns as you want that fit what you're given.

An alternative example would be that the sequence repeats the numbers 4, 9, 28, 36, 108, 0 over and over again. And there are infinitely many other possibilities.

It is also definite that there will be an expression of the form ax³ + bx² + cx + d that will express the xth term of the sequence. Unfortunately, I have no idea how to start working that out.

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Why did the vector cross the road?
It wanted to be normal.

Yoyodragon

Guest

Re: help on strange series!

Hi John,

Thank you for your answer, it sounds reasonable.