# Math Help - Urgent help needed

1. ## 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.

2. Tn = a(r)^n-1

Where N is the term in the equation.

3. 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?

4. Arithmetic Progression
$a_{n}=a_{1}+(n-1)d$

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

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

a = first term
n = nth term of the sequence
r = common ratio

5. 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

6. 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

7. 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: $\frac{\text{n(n+1)}}{2}$

8. ok so im not sure if i used that formula right, i tried it and it didnt add up.

9. 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

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