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

    14
    ReputationRep:
    https://cdn.discordapp.com/attachmen...60/unknown.png
    https://cdn.discordapp.com/attachmen...01/unknown.png


    I tried to do a substitution with 2 equations but i got a really horrible quadratic with massive numbers and of course it didn't factorise but when i came to use the formula on it, it turned out the formula didn't work anyway.


    How am i supposed to approach this question? The second part finding K and r
    • Study Helper
    Online

    13
    (Original post by will'o'wisp2)
    https://cdn.discordapp.com/attachmen...60/unknown.png
    https://cdn.discordapp.com/attachmen...01/unknown.png


    I tried to do a substitution with 2 equations but i got a really horrible quadratic with massive numbers and of course it didn't factorise but when i came to use the formula on it, it turned out the formula didn't work anyway.


    How am i supposed to approach this question? The second part finding K and r
    Given the answers are to 1 dec.pl, I wouldn't expect the quadratic to factorise.

    I think it's just a case of ploughing through with the formula - which did work when I tried. I wasn't being careful, and there may be an error. I ended up with

    15r^2-110r+104=0

    Arithmetic slip somewhere?
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    Given the answers are to 1 dec.pl, I wouldn't expect the quadratic to factorise.

    I think it's just a case of ploughing through with the formula - which did work when I tried. I wasn't being careful, and there may be an error. I ended up with

    15r^2-110r+104=0

    Arithmetic slip somewhere?
    hmm, let me try again i'll report back asap
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    Given the answers are to 1 dec.pl, I wouldn't expect the quadratic to factorise.

    I think it's just a case of ploughing through with the formula - which did work when I tried. I wasn't being careful, and there may be an error. I ended up with

    15r^2-110r+104=0

    Arithmetic slip somewhere?
    man i get 24r^2  -310 +351

    but nvm it's ok forget this one for now, i'll let it rest
    • Study Helper
    Online

    13
    (Original post by will'o'wisp2)
    man i get 24r^2  -310 +351

    but nvm it's ok forget this one for now, i'll let it rest
    Just worked out a different route, avoiding the quadratic.

    In order to avoid the subscripts, I'll use T for P_{n+1} and S for P_n

    Our formula then becomes:

    \displaystyle T=\frac{KrS}{K+(r-1)S}

    Which after multplying up, and grouping the K terms together, gives:

    \displaystyle (T-rS)K = (1-r)ST

    Plug in the two sets of values for S and T to get two equations, in K and r.

    Then divide one equation by the other, eliminating K and the 1-r, and after multiplying up, you're left with a linear equation in r.
 
 
 
  • 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
    Did TEF Bronze Award affect your UCAS choices?
    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.