newton-raphson please help

Question

Yusuf 2014-1-18

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/112831-newton-raphson-please-help

回答： Meysam Mahooti 2019-12-5

i want to find roots of f(x)=x(x-1)e^x by using newton raphson but instead of finding df/dx by using code , i want to use central numerical differentiation which is that f_diff(x)=(f(x+h)-f(x-h))/2*h; and h is any variable but smaller is better such as 10^-6 . and i want write each of them as a function file then call just newton-raphson formule and find root.

please help me

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Answer 1

Amit 2014-1-18

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https://ww2.mathworks.cn/matlabcentral/answers/112831-newton-raphson-please-help#answer_121279

在 MATLAB Online 中打开

function ra = myfunc(x)
% x the input
F = x*(x-1)*(e^x);

and seperately,

functin dif = mydiff(x,h)
dif = (myfunc(x+h)-myfunc(x-h))/(2*h)

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Yusuf 2014-1-18

i tried this code but did not work

in order to find roots of F, i need to use x(k+1)=x(k)-[f(x)/f_diff(x)] , i create another function NR = Newton_Root(x0,h,M) %h is the value that uses in mydiff % M is number of iteration % x0 initial value x=x0 for i=0:M x(i+1)=x(i) x(i+1)=x(i)-[myfunc(x)/mydiff(x)] end end

Amit 2014-1-18

What error you got?

One thing I see is, while using mydiff, you have just used mydiff(x), however it should be mydiff(x,h)

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Answer 2

Mischa Kim 2014-1-18

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/112831-newton-raphson-please-help#answer_121309

在 MATLAB Online 中打开

Try:

function [x, fx] = my_NR(my_tol, h, x0)
x = x0;
while (abs(f(x)) > my_tol)
    dx = f(x)/df(x, h);
    x  = x - dx;
end
fx = f(x);
end
function f_val = f(x)
   f_val  = x*(x - 1)*exp(x);
end
function df_val = df(x, h)
   df_val = (f(x + h) - f(x - h))/(2*h);
end