Originally Posted by

**Lammalord** okay, i cant reamber how to make the equation for this problem and i need help:

okay it starts a 0, then the first number is 3000, second, 9000, third, 18000, 4th 30000 so on..

so it increases by 3000 for each time it adds, but i cant reamber how to make the problem, that was like 2 years agao

Hi,

I rewrite your problem using a table:

Code:

n a_n factorized
1 3000 = 1 * 3000
2 9000 = 3 * 3000
3 18000 = 6 * 3000
4 30000 = 10 * 3000
... ...... ......

In general you need an equation : a_n = f * 3000, where f is a factor with respect to n.

As you may have noticed the factor is exactly the sum of the n:

Code:

n sum
1 1
2 1+2 = 3
3 3+3 = 6
4 6+4 = 10
... ...

The sum of the first n positive integers is calculated by: S = (1/2)n(n+1)

So your equation becomes:

a_n = (1/2)n(n+1)*3000

EB