Evaluating derivative using symbolic toolbox

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WILLIAM WENGER
WILLIAM WENGER 2022-2-9
评论： WILLIAM WENGER 2022-2-11
Hello all, I am trying to evaluate a derivative using the symbolic tool box.
I chose a simpler exquation to show, but if I can get this to work my code will work.
Here's the issue:
syms x
f_of_x = x^2 * sin(x)
der = diff(f_of_x)
der_2 = der(2)
As you can see I'm trying to evaluate the function @x = 2; but I'm getting the error "index exceeds number of array elements"
I checked the documentation and I literally abandoned my code, and wrote the example code direct from the documentation, still doesn't work.

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采纳的回答

Paul
Paul 2022-2-10
Ran in:
Two ways to do this:
syms x
f_of_x = x^2 * sin(x);
der = diff(f_of_x,x) % I like to be explicit on the variable of differentiation
der = 
der_2 = subs(der,x,2)
der_2 = 
Or
f(x) = x^2 * sin(x);
der(x) = diff(f(x),x)
der(x) = 
der_2 = der(2)
der_2 = 
  2 个评论
Paul
Paul 2022-2-10
Ran in:
"Thank you! For the second example does that work because of the parenthesis on f(x)?"
Yes. f_of_x and f(x) are different types of objects:
syms x
f_of_x = x^2 * sin(x);
f(x) = x^2 * sin(x);
whos
Name Size Bytes Class Attributes f 1x1 8 symfun f_of_x 1x1 8 sym x 1x1 8 sym
A symfun works very similarly to a mathemetical function. I think there was a thread here not too long ago that discussed differences between sym and symfun objects, but I can't find it.
WILLIAM WENGER
WILLIAM WENGER 2022-2-11
I’ll look for another thread, or the documentation. Thanks!

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更多回答（1 个）

WILLIAM WENGER
WILLIAM WENGER 2022-2-10
Thank you! For the second example does that work because of the parenthesis on f(x)?

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