Turn on thread page Beta

Problems involving Arithmetic Progressions watch

    • Thread Starter
    Offline

    19
    ReputationRep:
    4. The first, second and fourth terms of an Arithmetic Progression are x + 1, 2x - 1, and 2x + 5.

    Sn formula is Sn = 1/2n(2a+d(n-1))

    I'm completely lost...
    a = x + 1

    2nd term = a + d
    2x - 1 = x + 1
    x = 2 but it says x = 5
    Then others I'm clueless how to find.
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by ckfeister)
    4. The first, second and fourth terms of an Arithmetic Progression are x + 1, 2x - 1, and 2x + 5.

    Sn formula is Sn = 1/2n(2a+d(n-1))

    I'm completely lost...
    a = x + 1

    2nd term = a + d
    2x - 1 = x + 1
    x = 2 but it says x = 5
    Then others I'm clueless how to find.
    What is your d? You didn't include that in the bold line. Find d by doing (2x-1)-(x+1)=x-2 and then you may proceed.

    Also what are you trying to find? If it's not the sum then that formula is useless.
    • Thread Starter
    Offline

    19
    ReputationRep:
    (Original post by RDKGames)
    What is your d? You didn't include that in the bold line. Find d by doing (2x-1)-(x+1)=x-2 and then you may proceed.

    Also what are you trying to find? If it's not the sum then that formula is useless.
    I have to find x,a,d
    x = 5, a = 6, d = 3, but don't know how to find them.
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by ckfeister)
    I have to find x,a,d
    x = 5, a = 6, d = 3, but don't know how to find them.
    So
    a_1=x+1
    a_2=2x-1

    so we get the difference in terms of x which is x-2

    Then express the fourth term in terms of the first one and a certain multiple of the difference.
    • Thread Starter
    Offline

    19
    ReputationRep:
    (Original post by RDKGames)
    So
    a_1=x+1
    a_2=2x-1

    so we get the difference in terms of x which is x-2

    Then express the fourth term in terms of the first one and a certain multiple of the difference.
    What did you remove? I was eating came back and it was gone.
    • Thread Starter
    Offline

    19
    ReputationRep:
    bump
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by ckfeister)
    What did you remove? I was eating came back and it was gone.
    Error.

    Simply express the fourth term since you know the formula u_n=a+(n-1)d and solve for x.
    • Thread Starter
    Offline

    19
    ReputationRep:
    (Original post by RDKGames)
    Error.

    Simply express the fourth term since you know the formula u_n=a+(n-1)d and solve for x.
    Ohhh I've been using Sn not Un, thanks.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: November 12, 2016
The home of Results and Clearing

2,646

people online now

1,567,000

students helped last year

University open days

  1. Bournemouth University
    Clearing Open Day Undergraduate
    Wed, 22 Aug '18
  2. University of Buckingham
    Postgraduate Open Evening Postgraduate
    Thu, 23 Aug '18
  3. University of Glasgow
    All Subjects Undergraduate
    Tue, 28 Aug '18
Poll
How are you feeling about GCSE results day?
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.