Finding the derivative of a function with respect to the derivative of a variable

I am trying to derive the expression that contains a dependent variable x with respect to the derivative of x, but although I defined dx as a variable I get the error message

Second argument must be a variable, a symfun or a list of variables and symfuns.

The output should be 2*diff(x,t,1).

syms x(t)  
dx = diff(x,t,1);   
L = dx^2
functionalDerivative(L,dx)

Answer 1

[Deepak Meena]

Hi Andrew ,

Walter Robinson has beautifully explained why there is problem with using diff(f,diff()) here.

Now to get the desired Output you can do get something like this:

syms x(t)  dx ad
dx = diff (x, t, 1);   
L(ad) = subs(dx^2, dx, ad);
p = subs(diff(L, ad), ad, dx);
>>p
p(ad) =
2*diff(x(t), t)

Thanks

Deepak