Results are out! Find what you need...fast. Get quick advice or join the chat
x

Unlock these great extras with your FREE membership

  • One-on-one advice about results day and Clearing
  • Free access to our personal statement wizard
  • Customise TSR to suit how you want to use it

c2 geometric series

Announcements Posted on
Find your uni forum to get talking to other applicants, existing students and your future course-mates 27-07-2015
Competition: win a karting session for you and seven mates! 24-07-2015
  1. Offline

    ReputationRep:
    a geometric progression has a positive common ratio, its first terms are 32,b,12.5 how do you find the value of b
    could you please show step by step instructions so i know what technique to use when answering similar questions
  2. Offline

    ReputationRep:
    If you know two consecutive terms of a gp you can find the value of r by dividing
  3. Offline

    ReputationRep:
    (Original post by the bear)
    If you know two consecutive terms of a gp you can find the value of r by dividing
    they arent consecutive
  4. Offline

    ReputationRep:
    Sorry i misread 12.5 as 12, 5...

    Instead say

    b/32 = 12.5/b..... both sides are the ratio r

    solve for b
  5. Offline

    ReputationRep:
    (Original post by anisah4295)
    they arent consecutive
    From your post, it implies they are. Is the first term 32, with the second being b and the third being 12.5? Consecutive means 'in order', so 1, 2, 3 or similar.
  6. Offline

    ReputationRep:
    Is it necessary to know how to prove the formula of sum to infinity in C2?
  7. Offline

    ReputationRep:
    (Original post by Buongiorno)
    Is it necessary to know how to prove the formula of sum to infinity in C2?
    no, you only need to know the formula
  8. Offline

    ReputationRep:
    write the series down and under the first term write A, AR under the second, AR^2 under the third and so on. you have the first and third terms so that A and AR^2, so divie the third by the first and you're left R^2. square root and you have the common ratio
  9. Offline

    ReputationRep:
    But why use a formula if you don't know how it works? Waste of math in my opinion! And the formula is dead simple!

    Ok, so take the sum of n terms of a sequence starting with a and common factor r.

    Sn = a(r^n -1)/(r-1)

    Multiply top and bottom by (-1), Sn = a(1 - r^n)/(1 - r)

    Now think about it... what happens when n goes to infinity? Well... if r>1, then r^n gets bigger and bigger to infinity. Same thing when r<(-1) (but negative), so Sn goes to infinity!

    Now what happens if r=1? It becomes a(1-1)/(1-1). But wait... we can't divide by 0! So this doesn't work.
    With r=(-1), this gets really weird, because when n is even, it becomes a(1-1)/(1- (-1)) = 0.
    But when n is odd, this becomes, a(1-(-1))/(1-(-1)) = a. But which is infinity? Even, or odd? So we don't really know. This is undefined.

    What if -1<r<1 ? Well... then r>r^2>r^3>....r^n>...., so when n goes to infinity, r^n becomes 0! :O If we plug that in, we get Sn = a(1-0)/(1-r) = a/(1-r)

    Therefore, when |r|<1, Sn = a/(1-r).



    See the truth is, if you don't understand what you're doing in math, you WILL get a LOWER grade, and you WILL have trouble. For example, I don't have to memorize formulas like that, because I know how to prove it! So if I REALLY get stuck, I can just quickly prove it, half in my head, half on paper, in like 2 mins. And then you'll use the formulas that you got many times and eventually... POOF! Its hammered in your subconscious. And this takes a LOT longer to happen, if you just try to learn the formula.

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide to join now Processing…

Updated: April 26, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Poll
Have you ever posted a picture of your food on social media?
Results and Clearing

SQA results chat

Come talk about your results here

new on tsr

Join the American Society

Whether you're from the USA or just love it!

Study resources
x

Think you'll be in clearing or adjustment?

Hear direct from unis that want to talk to you

Get email alerts for university course places that match your subjects and grades. Just let us know what you're studying.

Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.