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

    0
    ReputationRep:
    right guys i have the question- the second term of a geometric sequence is 8 and the fifth is -1, could anyone give me an equation i could use to find r and a please?
    • PS Helper
    Offline

    14
    PS Helper
    Well you know that, by definition of a geometric sequence, the nth term is given by u_n = ar^{n-1}. You also know that u_2=8 and u_5=-1. What do you think you can do with these pieces of information?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by nuodai)
    Well you know that, by definition of a geometric sequence, the nth term is given by u_n = ar^{n-1}. You also know that u_2=8 and u_5=-1. What do you think you can do with these pieces of information?
    i've treated 8 as term one for now, and -1 as term 4, and got the equation -1=8*r^3

    so therefore r=-0.5 and a=16, but when i try to prove it, i get the wrong answer, argh
    • PS Helper
    Offline

    14
    PS Helper
    (Original post by bambamty)
    i've treated 8 as term one for now, and -1 as term 4, and got the equation -1=8*r^3

    so therefore r=-0.5 and a=16, but when i try to prove it, i get the wrong answer, argh
    Your value for r is correct, but not a. How did you get your value for a?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by nuodai)
    Your value for r is correct, but not a. How did you get your value for a?
    i put it into the general equation so 8=a*0.5^1
    so 8/0.5 = a?
    • Thread Starter
    Offline

    0
    ReputationRep:
    hmm, i believe it would, i shall go back and re-check that question now :P
    • PS Helper
    Offline

    14
    PS Helper
    (Original post by bambamty)
    i put it into the general equation so 8=a*0.5^1
    so 8/0.5 = a?
    But r isn't 0.5 is it?
    Offline

    9
    ReputationRep:
    Hi.

    I am having difficulty with these questions too.

    Who else is finding core 2 dull compared to Core 1???
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by Mewzle)
    ar = 8
    ar4 = -1
    By dividing, r3 = -0.125, and so r = -0.5?
    i've got that now! wooo! i just mistyped it on here i think!
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: March 22, 2011
Poll
Do you agree with the proposed ban on plastic straws and cotton buds?
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.