# Sequence

Discussion in 'Community Discussion' started by Jasonbot, Nov 10, 2007.

1. ### Jasonbot macrumors 68020

Joined:
Aug 15, 2006
Location:
The Rainbow Nation RSA
#1
I'm having a little trouble finding the equation for this sequence:

2,-6,18,-54

Firstly I dont know how to express the change from negative to positive in the equation and secondly i'm not really getting the equation. help appreciated as i'm studying for my exam, thank

2. ### Queso macrumors G4

Joined:
Mar 4, 2006
#2
Well, the number is multiplied by -3 every time. Can't remember how to express it as it's a long time since I did any of this, but that's the sequence anyway

Could it be b=(-3)a or something like that?

3. ### siurpeeman macrumors 603

Joined:
Dec 2, 2006
Location:
the OC
#3
it's a geometric sequence with the ratio being -3 and the first term being 2.

g(n) = 2*(-3)^(n-1)

4. ### Jasonbot thread starter macrumors 68020

Joined:
Aug 15, 2006
Location:
The Rainbow Nation RSA
#4
oh, i forgot where to put the minus. but i see now. thanks!

5. ### siurpeeman macrumors 603

Joined:
Dec 2, 2006
Location:
the OC
#5
glad i could be of some help. i would have written more, but it was really really late, and i needed to get some sleep. anyway, if you have an alternating sequence, that can be obtained by (-1)^n or (-1)^n-1, depending on the sign of the first term. study well!

6. ### theman macrumors 6502a

Joined:
Jul 26, 2007
#6
ha, I remember all the series stuff...

I hope you don't have to learn Taylor series, that stuff was annoying as hell! Definitely the hardest subject from calculus last year.