Determine f(x)=0 with newtons method

Question

Joakim Karlsson 2017-11-4

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/365089-determine-f-x-0-with-newtons-method

评论： Matt J 2017-11-4

Hi everybody. I am going to be honest that this is something for school but I need some help and guidance with it. I am almost 100% done with the code but it somehows doesnt seem to work.

So the task is to determine f(x)=0 using newtons method and my function code looks like this

function x = min_newton(f,Df,x0,tol)
%Determine f(x)=0
%Code below
kmax=100;
x=x0
for k=1:kmax
  h=-f(x)/Df(x);
  x=x+h;
  if abs(h)<tol, break, end
end
end

It seems alright to me. Then my code below is to determine the f(x)=0 to a given function.

f=@(x)0.5.*(x-2).^2-2.*cos(2*x)-1.5;
Df=@(x)x-2.*cos(2*x)-3.5;
plot(x,f(x)) %intersects approx at x=-0.5 x=1, x=2, x=4
x0=-0.5;
zero=min_newton(f,Df,x0,0.5e-8);
disp(zero)

The problem for me here is whatever I set my x0 to it only calculates it for the first zero which is approx at -0.5 and if I set my x0=1 it still calculates for -0.5. What is wrong here exactly?

I have tried different kmax etc.