Loop structure for fixed point.

Hello, I have problem with Loop structure for fixed point.

This is the problem, and i made code like this.

It only caculates 1 time and just stops. Which mean, it's not repeating my code, xb=x; and x=sqrt(1.8*x+2.5)

if i manually repeat xb=x; and x=sqrt(1.8*x+2.5) the answer is correct, however it's not repeating those code.

What's wrong about my loop and How to fix it?

[Rik]

Please post your code as code. Also, why haven't you tried stepping through your code line by line to see what the variables are?

You can find guidelines for posting homework on this forum here. If you have trouble with Matlab basics you may consider doing the Onramp tutorial (which is provided for free by Mathworks). If your main issue is with understanding the underlying concept, you may consider re-reading the material you teacher provided and ask them for further clarification.

I hope @VBBV will not actually do as you ask and provide a working solution. If you hand that in as your own work you would be commiting academic fraud. Asking for help is fine, asking for solutions isn't.

[Rik]

Your question is asking for help, the comment you wrote was asking for a solution. I was not claiming you were doing something nefarious, I was simply pointing you to helpful guidelines and to a tutorial.

If you had actually stepped through your code, you would have realised the values of your variables:

x0=5;
x=sqrt(1.8*x0+2.5);
xb=x;
x=sqrt(1.8*x+2.5);
disp((x-xb)/x)

   -0.1561

The solution to your problem is that you probably want to look at the absolute value of the error term.

[형석 김]

@Rik i've actually put abs on the error, by the way thanks for the advice.

i'm not sure the only problem is the abs..

cause after i put abs, it was also littlebit different from the manually solved answer

[Rik]

How large is that difference? And what is the code you're currently using? And what results are you expecting?

[형석 김]

@Rik The value of x i got is 2.7892, and the correct answer when I calculated manually is 2.7424. I've also calculated error when x=2.7892 manually, but error sllightly exceeds 0.05, which is (2.9333-2.7892)/2.78792=0.05166.

Answer 1

[Walter Roberson]

and the correct answer when I calculated manually is 2.7424

No it is not. The correct answer is 2.71934053986603

format long g
roots([-1 1.8 2.5])

You tried the transform

-x^2 + 1.8*x + 2.5 == 0
implies 1.8*x + 2.5 == x^2
implies sqrt(1.8*x + 2.5) == sqrt(x^2)

and you deduced from that that

x == sqrt(1.8*x + 2.5)

but that is not correct. sqrt(x^2) is not x: it is abs(x) . So

abs(x) == sqrt(1.8*x + 2.5)

and you should be working both branches of that.

I suggest that you instead

-x^2 + 1.8*x + 2.5 == 0
implies 1.8*x == x^2 - 2.5
implies x == (x^2 - 2.5)/1.8

[형석 김]

Thanks for your advice.