How to solve f'(x)=0?

I want to find f'(x)=0

My function is f(x)=2x^2+4x. Then in MATLAB I wrote f2=diff(f);

Next step is to calculate f'(x)=0, but I do not know how. I tried a few examples I found with fzero() but it did not work.

Thanks!

[Ela Markovic]

How did you define your function f(x)=2x^2+4x?

Similar question was already answered here ("How to find the derivative of the function at some value of x?"), look for answer by John D'Errico. Basically, MATLAB cannot do symbolic differentiation. You can differentiate in advance and pass that to function.

On the other hand, If you have Symbolic Toolbox installed and licensed only then can you use symbolic functions syms.

[laros]

I want to find the answer numerically. The question you posted is about value of x, I already figure that out. But I'm now trying to figure out how to solve f'(x)=0.(not symbolically).

[Ela Markovic]

How about:

fun = @f; % function
x0start = 2; % initial point
x0 = fzero(fun,x0start);
disp(x0)

    -1

function y = f(x)
    y = 4*x+4;
end

Answer 1

[Torsten]

You want to find x symbolically or numerically ?

Numerically use "fzero".

Symbolically use "solve".

[laros]

I want to find x numerically and I want to find it by using double.

Do I need to first use fzero and then 'convert' it by using double?

[Torsten]

If you use "fzero", you don't need to convert anything. "fzero" works in doubles.

If you have problems differentiating f(x) = 2*x^2+4*x and want MATLAB do it for you, I suggest

syms x
f = 2*x^2+4*x;
df = diff(f,x);
fun = matlabFunction(df);
x0 = 2;
x = fzero(fun,x0)

x = -1

[Jawad]

why are you using x0 function?

[Jawad]

what does it do

[Torsten]

You must supply a value where "fzero" starts searching for a solution of f'(x) = 0. I chose x0 = 2. You may choose another value if you like.