Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    Find the sum of:

    41 + 38 + 35 + ... + (-19)

    I thought I would use the formula 0.5n(a+l) but I don't get the same answer which the book gives. I must be doing something wrong- help please! The answer is 231.

    and then... the first term is -11 and the ninth term of the series is 1
    Find the common diffrence and teh sum of the first twenty terms of teh series.

    Any explanation would be appreciated. I've managed to answer all of the questions without negative numbers- so it's got to be there that I'm going wrong. Thanks
    Offline

    14
    ReputationRep:
    (Original post by Elm_lily)
    Find the sum of:

    41 + 38 + 35 + ... + (-19)

    I thought I would use the formula 0.5n(a+l) but I don't get the same answer which the book gives. I must be doing something wrong- help please! The answer is 231.

    and then... the first term is -11 and the ninth term of the series is 1
    Find the common diffrence and teh sum of the first twenty terms of teh series.

    Any explanation would be appreciated. I've managed to answer all of the questions without negative numbers- so it's got to be there that I'm going wrong. Thanks

    I expect you got the number of terms wrong. Consider 41, 38, 35,32
    Difference = 9
    9 divided by 3 =3
    Number of terms = 3+1=4
    Offline

    14
    ReputationRep:
    It's an arithmetic series, not a geometric...
    Offline

    14
    ReputationRep:
    (Original post by maxfire)
    It's an arithmetic series, not a geometric...
    Neither the OP or I have suggested it is geometric. I don't see why you think otherwise.
    Offline

    14
    ReputationRep:
    (Original post by steve2005)
    Neither the OP or I have suggested it is geometric. I don't see why you think otherwise.
    Didn't read the question properly did I?
    Spoiler:
    Show
    :getmecoat:
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: February 14, 2010

University open days

  • University of Exeter
    Undergraduate Open Days - Exeter Campus Undergraduate
    Wed, 24 Oct '18
  • University of Bradford
    Faculty of Health Studies Postgraduate
    Wed, 24 Oct '18
  • Northumbria University
    All faculties Undergraduate
    Wed, 24 Oct '18
Poll
Do protests make a difference in political decisions?
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

Equations

Best calculators for A level Maths

Tips on which model to get

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.