How to compute a derivative of a function and to solve it with f'(x)=0 !

Hi there!

I'm working on a simple project in order to learn how to correctly use Matlab since a few days and I've a little problem with an optimisation process:

I've this function, very simple one: f(x)= x² -2x -3 I would like to compute the derivative of this function, therefore I use the function "diff" of Matlab in this way:

f =x^2 - 2*x - 3
diff(f)

f=
2*x - 2

then I'd like to set this derivative equal to 0 in order to find x:

f(x) = 2*x - 2 = 0  <=>  x = 1

But I wish to do it automatically, without typing the computed derivative in Matlab everytime. I actually tried several times with fzero but it never worked...

f =x^2 - 2*x - 3
f2 = diff(f)
fzero(f2,0)

??? Undefined function or method 'isfinite' for input arguments of
type 'sym'.

Error in ==> fzero at 317
    elseif ~isfinite(fx) || ~isreal(fx)

Do you have any idea how can I write a mat file (or function file I can't remember the expression again ;)) in order to compute derivative and this derivative equal to 0 automatically? I hope this is understable, sorry for my mistakes.

yours Aurélien

Answer 1

[Walter Roberson]

solve(f2,x)

Or alternately,

f3 = matlabFunction(f2);
fzero(f3, 0)

fzero() takes a function handle. A symbolic expression is not a function handle.