Hey there! Sign in to join this conversationNew here? Join for free
Turn on thread page Beta
    • Thread Starter
    Offline

    8
    ReputationRep:
    What is the 7th term of a GP if the difference between between it's 3rd term and 1st termis 18 and that between it's 4th and 2nd is 9?
    Offline

    9
    ReputationRep:
    (Original post by a_09)
    What is the 7th term of a GP if the difference between between it's 3rd term and 1st termis 18 and that between it's 4th and 2nd is 9?
    What have you tried?

    The nth term of a GP is ar^{n-1} where a is the first term and r is the common ratio.
    • Thread Starter
    Offline

    8
    ReputationRep:
    (Original post by BobbJo)
    What have you tried?

    The nth term of a GP is ar^{n-1} where a is the first term and r is the common ratio.
    I've just tried to make these equations:-
    a(r)^2 - a=18 ----(i)
    ar - ar^3=9 ----(ii)
    But I don't know what to do next 😓
    Offline

    9
    ReputationRep:
    (Original post by a_09)
    I've just tried to make these equations:-
    a(r)^2 - a=18 ----(i)
    ar - ar^3=9 ----(ii)
    But I don't know what to do next 😓
    (i) can be rewritten as a(r^2-1)=18
    (ii) can be rewritten as ar(1-r^2)=9 or -ar(r^2-1)=9

    Can you see what the next step should be?
    • Thread Starter
    Offline

    8
    ReputationRep:
    (Original post by BobbJo)
    (i) can be rewritten as a(r^2-1)=18
    (ii) can be rewritten as ar(1-r^2)=9 or -ar(r^2-1)=9

    Can you see what the next step should be?
    Okay, I'm trying.
    • Thread Starter
    Offline

    8
    ReputationRep:
    Oh yess! I got it! Thanks a loads!! 😃
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: February 7, 2018
Poll
“Yanny” or “Laurel”
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

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

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

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