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
    17,720
    4,788
    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?

    Your equation is

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

    Remember your order of operations.

    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
    17,720
    4,788
    Now how about the part about showing how you got the answer you did so that we have something to start with.

    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
    17,720
    4,788
    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?
     
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