Just one sec...
Hey! Sign in to get help with your study questionsNew here? Join for free to post

c2 geometric series

Announcements Posted on
Help us uncover the truth about exam cheats – is cheating ever ok? What if you wouldn't get caught? 27-06-2016
    • Thread Starter
    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
    Online

    ReputationRep:
    If you know two consecutive terms of a gp you can find the value of r by dividing
    • Thread Starter
    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
    Online

    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
    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.
    Offline

    ReputationRep:
    Is it necessary to know how to prove the formula of sum to infinity in C2?
    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
    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
    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. Oops, you need to agree to our Ts&Cs to register
  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
Do you think the next leader of the Conservative party should call a General Election
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read here first

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22

Registered Office: International House, Queens Road, Brighton, BN1 3XE

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