[求助]consider the following function这个题目

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我下了下面的function, 之后要写个M-files 但是M-files出错，麻烦高手帮我检查下。。。

function root = newraph(f, df, xi, precision)

% INPUTS:
%  - f: function handle of the equation to be solved
%  - df: function handle of the derivative of the equation to be solved
%  - xi: the initial guess / the next guess x_i_+_1
%  - precision: stopping criteria determined by the user
% OUTPUT:
%  - root: the root of the equation

% Calculate f(xi) and f'(xi)
fxi = f(xi);
dfxi = df(xi);

while abs(fxi) > precision

 xi = xi - fxi/dfxi;    % Calculate the new estimated root, x_i_+_1
    fxi = f(xi);           % Re-calculate f(x_i_+_1)
    dfxi = df(xi);         % Re-calculate f'(x_i_+_1)

end

% The final xr value is the root
root = xi;

麻烦帮我检查下M-files

% function
% f = x.^3-8*x.^2-200*x+1000
% f'= 3*x.^2-16*x-200

% input   x[-10,10]
% xi = -10;
% precision = 0.0001;

%  root = newraph(f, df, xi, precision)
root = newraph( f, 3*xi.^2-16*xi-200, -10, 0.0001);

fprintf('The root of the equation is %.4fn',root)

问题：

??? Undefined function or variable 'xi'.

Error in ==> Lab6t4 at 17
root = newraph( f,3*xi.^2-16*xi-200,-10, 0.0001);

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