# Geometric progression

Discussion in 'Math' started by Julia.S, Nov 19, 2012.

1. ### Julia.S Thread Starter New Member

Jul 21, 2012
9
0
Hello,

How can I calculate an for the following:

a1=a
a2=1+(-0.5)p
a3=1+(-0.5)+[(-0.5)^2]p
a4=1+(-0.5)+[(-0.5)^2]+[(-0.5)^3]p
...
...

I got
an=[(-0.5)^n-1]p+ ??????

10x!

2. ### WBahn Moderator

Mar 31, 2012
17,457
4,701
I don't understand your setup. You have a1=a. What is 'a'? Your equation for an doesn't include 'a' at all, so how can it possibly produce a1=a?

an = [ {(-0.5)^n} - 1]p

Clean things up and walk through how you got your answer. Remember, it doesn't matter if you are right or wrong, what is important is that you show some work that will let us start to understand your reasoning process so that we can spot where you are going right and where you are going astray.

Post that and then we will go from there.

3. ### Julia.S Thread Starter New Member

Jul 21, 2012
9
0
Hey,

Sorry, typing error

a1=P

10x!

4. ### WBahn Moderator

Mar 31, 2012
17,457
4,701

If nothing else, make some observations about how the series (and it is a geometric series, not a geometric progression, BTW) is similar and how it is different from other geometric series you have dealt with.

5. ### Julia.S Thread Starter New Member

Jul 21, 2012
9
0
Well, I noticed that I have [(-0.5)^n-1]P by observing the elements. As for the other part I noticed having 1+something (except in a1) and each ime i have addition of (-0.5).

10x!

6. ### WBahn Moderator

Mar 31, 2012
17,457
4,701
Okay, so lets look as see what the first few values of this expression are:

 n (-0.5)^n [(-0.5)^n-1] term 0 1 0 0 1 -0.5 -1.5 -1.5P 2 0.25 -0.75 -0.75P 3 -0.125 -1.125 -1.125P

Does this look like the pattern you are trying to construct?