Sequence

[Jasonbot]

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

 

[Queso]

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?

 

[siurpeeman]

it's a geometric sequence with the ratio being -3 and the first term being 2.

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

 

[Jasonbot]

it's a geometric sequence with the ratio being -3 and the first term being 2.

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

oh, i forgot where to put the minus. but i see now. thanks!

 

[siurpeeman]

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!

 

[theman]

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.