Hey there! Sign in to join this conversationNew here? Join for free

C3 Edexcel, Proving degree of accuracy via iterative formula Watch

    • Thread Starter

    Hey guys,
    A question that seems to pop up a lot on the past papers I have been given is giving proof to the degree of accuracy of 'x' to say 3 significant figures.

    The general layout of the questions is usually the iterative formula and they give you a starting 'x' value to substitute into the formula until you converge to the degree of accuracy they've stated so let's say I've got 'x' to 1.61 but I have to then proof that it's not 1.60 or 1.62 by using the values 1.605 and 1.615. e.g.

    Xn+1 = 12/7*ln(2Xn+5) -2

    'n' and 'n+1' are notations.

    So basically if I want to prove it's 1.61 I do this :

    1.605 - 12/7*ln(2*1.605 + 5) + 2 = around zero
    and the same for 1.615, but when I do this I never get the same answer as they do on the mark scheme but conveniently enough I always get the correct value closer to zero to prove the degree of accuracy of my answer... Basically is the way I'm doing this wrong or is there another way?

    I get for x=1.605 , -4.176*10^-3

    and for x=1.615, 1.653*10^-3

    but mark scheme says for 1.605 , −0.0073 and for 1.615, 0.0029

    Thanks Guys.

    You substitute the two numbers (1.605 and 1.615) into the original equation (f(x) , or y = ..), not the iterative formula. You should get two results, one a positive and one a negative, this therefore shows that the 1.61 is a root to 3 decimal places.
  • 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
    Brexit voters: Do you stand by your vote?
    Useful resources

    Make your revision easier


    Maths Forum posting guidelines

    Not sure where to post? Read the updated guidelines here


    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

    Write a reply...
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.