# How to substitute a function after differentiation?

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AntonioMG_MX on 15 Sep 2013
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!

Azzi Abdelmalek on 15 Sep 2013
Edited: Azzi Abdelmalek on 15 Sep 2013
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)
##### 2 CommentsShowHide 1 older comment
Azzi Abdelmalek on 15 Sep 2013
I'am not sure if it's possible or not

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