Patrik,

Works fine and very clear. How can I create a function out of your example so I can reuse it later? Do I need to use Module or With?

[
a = 6;

b = 3;

c = 2;

d = 1;

e = 3.2;

f[x_] := 2 a x^6 + b x^3 + c x^2 + d x + e x^3;

NSolve[f[x]/f'[x] == 0, x]

]

Thank you for your great explanations

Hugh

Patrik,

thank you for the example. With NSolve how can I select the Real value(s) that are returned, so i can pass this returned value to other functions?

thanks

Hugh

Hi Hugh!

I'm not a 100% sure that I understand what problem you are trying to solve using your for-loop. Are you trying to iteratively solve for a Real x such that f(x)/f'(x) = 0?

If that is the case, you could always use Solve or NSolve:

a = 6;
b = 3;
c = 2;
d = 1;
e = 3.2;
f[x_] := 2 a x^6 + b x^3 + c x^2 + d x + e x^3;
NSolve[f[x]/f'[x] == 0, x]

This will give you the following six solutions:

{{x -> 0.531951 + 0.688517 I}, {x -> 
   0.531951 - 0.688517 I}, {x -> -0.76146}, {x -> -0.151221 + 
    0.34885 I}, {x -> -0.151221 - 0.34885 I}, {x -> 0.}}    

If you absolutely necessarily want to do it with a loop you could write it like this:

maximumIterations = 100;
pr = prnew = -1;
i = 1;
While[i <  maximumIterations,
 prnew = pr - f[pr]/f'[pr]; i++; If[Abs[prnew - pr] < 10^-3, Break[]];
  pr = prnew]
prnew

Which will give you one solution, depending on your start-value for pr:

-0.76146

Hope that helps Hugh. And if not, if you could put your problem in context, I would gladly help you more. Have a nice day!

Patrik

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