trouble of taking derivative of function in newton method

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kingsley 2017-5-13

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https://ww2.mathworks.cn/matlabcentral/answers/340107-trouble-of-taking-derivative-of-function-in-newton-method

编辑： kingsley 2017-5-13

I'm trying to write a code for newton method. But I'm having a trouble of taking the derivative of the function(f(x(i))). Here is the code I have:

function [root]=newton_method(f, xi, tol, maxn)  % f is function handle
syms x
   for i= 1:maxn
   fprime = diff(f(x(i)),xi);
    root=xi-f(x(i)/fprime);
    if abs (x(i)-root)< tol
        root=x(i);
     break
    end 
   end
end

Can someone tell me how to fix it?

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John D'Errico 2017-5-13

编辑：John D'Errico 2017-5-13

What is f? Tell us CLEARLY what is f. In MATLAB terms, how is f represented? Is it symbolic? Is it a function handle?

Now, tell us what f(xi) is. Is it a scalar, double precision number, so a constant?

What is the derivative of a constant?

You cannot differentiate a constant. You CAN differentiate a function, IF it is stored in symbolic form. Then you can evaluate that result at the point xi.

So the point is, you need to understand that while you think of f as a function, it may be represented in MATLAB in several different ways. But you are the one who needs to use it and work with it.

kingsley 2017-5-13