Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    2
    ReputationRep:
    Hi. Help on this question, can't seem to get it right.

    The second term of a geometric series of positive terms is 100 and the fourth term is 64. Find the number of terms that must be added if the total is to exceed four times the first term.

    I have worked out a= 125, r=0.8 and the total must be more than 500. Substitute into formula sn = a(1-r^n)/1-r and taking logs but I can't get the right answer.

    Could someone please show me how to do this formula as that is the place I think I'm going wrong.

    Thanks.
    Offline

    11
    ReputationRep:
    (Original post by HumiT)
    Hi. Help on this question, can't seem to get it right.

    The second term of a geometric series of positive terms is 100 and the fourth term is 64. Find the number of terms that must be added if the total is to exceed four times the first term.

    I have worked out a= 125, r=0.8 and the total must be more than 500. Substitute into formula sn = a(1-r^n)/1-r and taking logs but I can't get the right answer.

    Could someone please show me how to do this formula as that is the place I think I'm going wrong.

    Thanks.
    I can't find a mistake in your working unless you post it
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by joostan)
    I can't find a mistake in your working unless you post it
    I just did my working out again...found my mistake. Thanks anyway
    Offline

    11
    ReputationRep:
    (Original post by HumiT)
    I just did my working out again...found my mistake. Thanks anyway
    Good
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Would you like to hibernate through the winter months?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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.