Solomon E paper question 4

Announcements Posted on
TSR looking different to you this week? Find out why here. 02-12-2016
    • Thread Starter
    Offline

    2
    ReputationRep:
    Stuck on C1 Solomon E paper question 4.

    A sequence of terms is defined by:

    Un = 3^n -2

    Write down the first four terms of this sequenc. Done

    b) The same sequence can also be defined by the recurrence relation

    Un 1+1 = aUn +b

    Find the vales of a and b.

    I got the first term correct - that being a+b = 7

    But the second term I got wrong - I got a quadratic and it should be

    7a +b = 25

    Not really sure how you get this .... I substitued what I got for u1 into it but as I say I get a quadractic in a....Help
    Offline

    3
    ReputationRep:
    (Original post by christinajane)
    ...
    So, when you plugged n=1 in, you got: (i.e: wherever you see n, you put a 1)

    u_{1+1} = au_{1} + b \iff 7 = a + b

    Now, when you plug n=2 in you get: (i.e: wherever you see n, you put a 2)

    u_{2+1} = au_{2} + b \iff u_3 = au_{2} + b

    But from the first part of the question, you know that u_3 = 3^{3} - 2= 27 - 2= 25 and u_2 = 7

    So: u_3 = au_2 + b\iff 25 = 7a + b
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by Zacken)
    So, when you plugged n=1 in, you got: (i.e: wherever you see n, you put a 1)

    u_{1+1} = au_{1} + b \iff 7 = a + b

    Now, when you plug n=2 in you get: (i.e: wherever you see n, you put a 2)

    u_{2+1} = au_{2} + b \iff u_3 = au_{2} + b

    But from the first part of the question, you know that u_3 = 3^{3} - 2= 27 - 2= 25 and u_2 = 7

    So: u_3 = au_2 + b\iff 25 = 7a + b


    Well - thats a bit of a tricky one...

    Basically what I did was

    U2 = a +b = 7 from the first bit

    Then for u3

    u3 = au2 +b

    u3 = a(a+b-7) +b

    :-/
    Offline

    3
    ReputationRep:
    (Original post by christinajane)
    Well - thats a bit of a tricky one...

    Basically what I did was

    U2 = a +b = 7 from the first bit

    Then for u3

    u3 = au2 +b

    u3 = a(a+b-7) +b

    :-/
    Well, that's valid - but, doesn't really help at all so there's no point doing it. I guess it'll come with practice; do note that Solomon papers are rather different to the actual exam, so don't be put off.
    • Thread Starter
    Offline

    2
    ReputationRep:
    Yeah paper E is the hardest one so far!!!

    At least what I did is kinda valid - its what I do for the other questions of this type - but obviously it wouldnt have worked out.

    I just knew I had to get a simultaneous equation - two linear ones not one linear and a quadratic!


    Solomons are very tricky - its tiny little details that keep catching me out - like the other one I just posted. I figured it out but it messed up the next bit of the question.

    I got x = 60 but coz its - x in the equation that actually means its -60! Which I didnt realise and messed the thing up - was only out by two though.

    Again my methods are there - just wrong numbers and signs :-/

    Hopefully the proper exam wont be AS tricky
    Offline

    3
    ReputationRep:
    (Original post by christinajane)
    Yeah paper E is the hardest one so far!!!

    At least what I did is kinda valid - its what I do for the other questions of this type - but obviously it wouldnt have worked out.

    I just knew I had to get a simultaneous equation - two linear ones not one linear and a quadratic!
    It's okay if you had a quadratic one, the only problem is that you get a(7a + b) + b but it's not equal to anything you know, so it's a bit useless in that sense.
 
 
 
Write a reply… Reply
Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: May 8, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Today on TSR
Poll
How are you feeling about doing A-levels?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read here first

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
Study resources

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.