Evaluating derivative using symbolic toolbox

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.

Answer 1

[Paul]

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 = 

[Paul]

"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]

I’ll look for another thread, or the documentation. Thanks!

Answer 2

[WILLIAM WENGER]

Thank you! For the second example does that work because of the parenthesis on f(x)?