Differentiating a Symbolic Function

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Ammar Taha
Ammar Taha 2021-4-9
评论： Star Strider 2021-4-9
If I have the following symbolic function:
syms f(x,y)
f(x,y) = 2*x^2 + y;
When I try to diff. this function w.r.t. (x) I get:
d = diff(f,x)
d(x, y) =
4*x
Which is a function in both x, and y NOT in (x) only.
How can I perform this and get the real exsisting function input only "i.e. (x) only here"?

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Star Strider
Star Strider 2021-4-9
That is the corrct result, since diff is taking the partial derivative with respect to ‘x’ only, and ‘y’ is considered a constant.
To get the derivatives of ‘f’ with respect to both variables, use the jacobian function:
jf = jacobian(f)
producing:
jf(x, y) =
[4*x, 1]
See Differentiation for more information.
Although not appropriate here, explore the functionalDerivative function.
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Chendi Lin
Chendi Lin 2021-4-9
Hi Ammar,
Without explicitly defining the differentiation variable, "diff" uses the default variable, which is "x" in your case.
To get both derivatives, you can do
[diff(f(x,y),x) diff(f(x,y),y)]
And this should give you the correct result.
Please refer to this document: Differentiate symbolic expression or function - MATLAB diff (mathworks.com)
Thanks.
CD

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