# Urgent help needed

• May 24th 2008, 02:24 PM
mathguy15
Urgent help needed
I have to get this done by monday and i am completely stumped on function rules. I seem to slightly remember there being a formula for finding a function rule, you know for finding the Nth term in a sequence and i dont remember it completely. Does anyone know this formula or if there is a formula? All help is greatly appreciated.
• May 24th 2008, 02:51 PM
gearshifter
Tn = a(r)^n-1

Where N is the term in the equation.
• May 24th 2008, 02:58 PM
mathguy15
Quote:

Originally Posted by gearshifter
Tn = a(r)^n-1

Where N is the term in the equation.

Ok so what does a and r equal?
• May 24th 2008, 03:10 PM
r_maths
Arithmetic Progression
$\displaystyle a_{n}=a_{1}+(n-1)d$

a = first term
n = nth term of the sequence
d = common difference

Geometric Progression
$\displaystyle a_{n}=ar^{n-1}$

a = first term
n = nth term of the sequence
r = common ratio
• May 24th 2008, 03:12 PM
gearshifter
A is the first term in the series.
R is the ratio of what you multiply every term by.

So for example A=1, R=0.5

If I wanted to look for term 6 then I'd use the formula:
Tn = a(r)^n-1

So you plug it in...

T6 = 1(0.5)^6-1
T6 = 1(0.5)^5
T6 = 0.03125
• May 24th 2008, 04:32 PM
mathguy15
Ok so im not really understanding you guys, want to try using it in the sequence that im trying to get it for? the sequence is 0, 1, 3, 6, 10, 15
• May 24th 2008, 04:50 PM
r_maths
Quote:

Originally Posted by mathguy15
Ok so im not really understanding you guys, want to try using it in the sequence that im trying to get it for? the sequence is 0, 1, 3, 6, 10, 15

You can't use AG/GP for that sequence which is why you're not getting it.

Formula you need is: $\displaystyle \frac{\text{n(n+1)}}{2}$
• May 24th 2008, 05:13 PM
mathguy15
ok so im not sure if i used that formula right, i tried it and it didnt add up.
• May 24th 2008, 09:41 PM
Isomorphism
Quote:

Originally Posted by mathguy15
ok so im not sure if i used that formula right, i tried it and it didnt add up.

Its just a little shifted... depends on how you start your counting. Mathematicians start from 0 (Rofl)

So now try $\displaystyle \frac{n(n-1)}2$, if you start from n=1.