You are Here: Home >< Maths

# Iteration watch

1. so an exam question has

2x^3 + 4x =3

and the question suggests you use an iterative form of

X = 3/4 - X^3 /2

Whereas I want to use

X= Cube root( 3/2 - 2x)

Their suggested form settles down and mine doesnt?
Taking a est between 0 and 1 why doesnt mine settle down?

Jj
2. (Original post by Jon J Wood)
so an exam question has

2x^3 + 4x =3

and the question suggests you use an iterative form of

X = 3/4 - X^3 /2

Whereas I want to use

X= Cube root( 3/2 - 2x)

Their suggested form settles down and mine doesnt?
Taking a est between 0 and 1 why doesnt mine settle down?

Jj
Whether or not an iteration formula x = g(x) converges or diverges depends on the magnitude of g'(x) (the gradient of g(x)) near the actual root. You can find an explanation here. I don't think this is within the A level syllabus, even for further maths.
3. (Original post by Jon J Wood)
so an exam question has

2x^3 + 4x =3

and the question suggests you use an iterative form of

X = 3/4 - X^3 /2

Whereas I want to use

X= Cube root( 3/2 - 2x)

Their suggested form settles down and mine doesnt?
Taking a est between 0 and 1 why doesnt mine settle down?

Jj
For GCSE you will always be given the iterative formula and starting value. All you need to be aware of is that a different formula and starting value can change whether the iteration works but you don’t need to know when/why.

Or do you know this but are still interested in when an iterative formula converges?

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.

This forum is supported by:
Updated: March 8, 2018
Today on TSR

Get the low down

### University open days

• University of Exeter
Wed, 24 Oct '18
Wed, 24 Oct '18
• Northumbria University
Wed, 24 Oct '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams