C2 GP

Maths and statistics discussion, revision, exam and homework help.

Announcements Posted on
Please change your TSR password 23-05-2013
Enter our travel-writing competition for the chance to win a Nikon 1 J3 camera 20-05-2013
IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
Search Thread
Advanced Search
Sign in to Reply
  1. Ganhad's Avatar
    1 10-05-2012  01:28
    • Exalted Member
    • Posts: 346
    C2 GP
    Find the number of terms in each of these geometric progressions

    3,6,12,...,768 well i just want to know how to do this then ill know all of em
  2. Killjoy-'s Avatar
    2 10-05-2012  01:38
    • Exalted and Worshipped Member
    • Posts: 1,083
    Re: C2 GP
    The general form of any term in a GP is ab^{r-1}, r is the term-to-term ratio, 2 in this case, and a must be 3.

    So set 768 to the equation for the general form and solve for n. (n is the term number of the last term in the sequence.)
    Last edited by Killjoy-; 10-05-2012 at 01:46.
  3. Ganhad's Avatar
    3 10-05-2012  01:40
    • Exalted Member
    • Posts: 346
    Re: C2 GP
    did that its not happening dont know how to get past that
  4. Killjoy-'s Avatar
    4 10-05-2012  01:42
    • Exalted and Worshipped Member
    • Posts: 1,083
    Re: C2 GP
    So 768=3 \times 2^{n-1}

    You'll need to take logs to base 2. (Or any base for that matter but 2 is simpler.)

    n-1=log_2(256)
  5. raheem94's Avatar
    5 10-05-2012  01:46
    • TSR Demigod
    • Posts: 5,512
    Re: C2 GP
    (Original post by Ganhad)
    did that its not happening dont know how to get past that
    768=3 \times 2^{n-1}

    Divide both sides by 3, 256 = 2^{n-1} \implies 256 = 2^n \times 2^{-1} \implies 256 = 2^n \times \frac12 \implies 512 = 2^n

    Now take logs.
Sign in to Reply
Search Thread
Advanced Search
Share this discussion:  
Useful resources

Articles:

Study Help rules and posting guidelinesStudy Help Member of the Month nominationsGuide to MathematicsMathematics resourcesMathematics websitesMathematics revision notes in the TSR wikiCambridge Assessment admissions tests (including STEP)How to use LaTeXCareers in Mathematics

Useful Dates:

Quick Link:

Unanswered Maths Threads

Groups associated with this forum:

View associated groups
Article updates
Moderators

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

This forum is moderated by:

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

Shortcuts

Get Started

TSR Group

Using TSR

Info

Connect with TSR

Reputation gems:
The Reputation gems seen here indicate how well reputed the user is, red gem indicate negative reputation and green indicates a good rep.
Post rating score:
These scores show if a post has been positively or negatively rated by our members.