Stuck at coding modified Newton-Rapson function

I made the function of Modified Newton-Rapson method .

But when I put function as 3*x^2-10*x+7, xi=0, error=0.01, and the result comes NaN.

What is wrong in this code. Please help.

function root=mnr(func, xi,error)

i=1;
xrold=0;
xr=xi
a=[];

fprintf('iteration root error\n')

while(1)

    xrold=xr;
    mul=subs(func,xr)/subs(diff(func),xr);
    xr=xr-subs(mul,xr)/subs(diff(mul),xr);
    a(i)=xr;
    er=(xr-xrold)/xr;

    fprintf(' %d %e %f\n', i, double(xr),double(er))
    i=i+1;
    if abs(er)<=error
        break
    end
end

Answer 1

[Torsten]

Why do you call it "Modified Newton-Raphson" ?

I do not quite understand what the two lines

mul=subs(func,xr)/subs(diff(func),xr);
xr=xr-subs(mul,xr)/subs(diff(mul),xr);

are supposed to do.

syms x
func = 3*x^2-10*x+7;
xi = 0;
error = 1e-6;
root = mnr(func, xi,error)

function root=mnr(func, xi,error)
syms x
df= diff(func,x);
i=1;
xrold=0;
xr=xi
a=[];
fprintf('iteration root error\n')
while(1)
    xrold=xr
    mul=double(subs(func,xr))/double(subs(df,xr));
    xr=xr-mul;
    %a(i)=xr;
    er=(xr-xrold)/xr;
    %fprintf(' %d %e %f\n', i, double(xr),double(er))
    %i=i+1;
    if abs(er)<=error
        break
    end
end
root = xr;
end

[seoung gi Sin]

Thanks for your help.