How to substitute a function after differentiation?

Hi everybody,

Suppose I do:

syms f(x,y)
df=diff(f,x)

df(x, y) =

D([1], f)(x, y)

Now I assign an expression to " f ", i.e.:

f(x,y) = y/x^2+x*y

f(x, y) =

x*y + y/x^2

My question is, how can I substitute " f " in " df "? The idea is to perform ther derivative automatically during substitution. Is this possible?

I have tried " subs ", " eval ", " compose ", direct substitution etc...

The problem seems to be that " df " has dependency on " x " and " y ", which makes sense, thus I am only able to make substitutions on " x " and " y " but " f ".

I know that I can assign the expression to " f " since the beginning, however that does not fit my application.

Thanks!

Answer 1

[Azzi Abdelmalek]

I think you can not make substitution, because the derivative is not yet calculated. what you have to do is

syms x y;
f=y/x^2+x*y;
df(x)=diff(f,x)

[AntonioMG_MX]

Thanks! My idea was to perform some operations with the derivative before derivating. Since this is possible in Maple, I thought it was possible also with Matlab too.

[Azzi Abdelmalek]

I'am not sure if it's possible or not