使用一个函数的输出作为另一个函数的输入

Question

Currently trying to write a function which is using the outputs of a previous function. The outputs are as follows:

f(x) = exp(x) -3*x.^2
fp(x) = exp(x) -6*x
fpp(x) = exp(x) -6

The new function is:

x_new = x - 2*(f(x))*(fp(x)) / 2*(fp(x).^2) -(f(x))*(fpp(x))

Tried to write [f,fp,fpp] = fun(x) because it worked for me in a script file earlier but now its saying

error not enough input arguments

Any ideas greatly appreciated.

The original question:

Answer 1

I don't know why it is not working but I think code below works well for me:

function[f,fp,fpp,x_new]=fun(x) 
f=exp(x) -3*x.^2 ;
fp=exp(x) -6*x ;
fpp=exp(x) -6;
x_new= x - 2*(f)*(fp) / 2*(fp.^2) -(f)*(fpp);
return


x=1234;
for ind=1:10
  [f,fp,fpp,x]=fun(x);
end

Answer 2

I'm not sure about the math here, but you need only one function, that should look something like this:

function r = halley(fun,x0,acc)
syms x
fp = diff(fun);
fpp = diff(fp);
x_new = symfun(x - 2*(fun(x))*(fp(x)) / 2*(fp(x).^2) -(fun(x))*(fpp(x)),x);
xold = x0;
xnew = x_new(xold);
while abs(fun(xold)) < acc && abs(xold-xnew) > acc
    tmp = xnew;
    xnew = x_new(xold);
    xold = tmp;
end
r = xnew;
end

and then you call it from a script with another function as input:

acc = 1.0e-8;
x0 = -5;
syms x
fun = symfun(exp(x) -3*x.^2 +1,x);
r = halley(fun,x0,acc)

however, in this exercise it says that the input of the function should be a function handle, like @fun , so maybe you should not use symbolic math? I hope this makes things clearer, though I don't know how this method should work.