Turn on thread page Beta

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.
Submit reply
Turn on thread page Beta
Updated: January 17, 2010

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
How are you feeling in the run-up to Results Day 2018?
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

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.