Define functions with independent variables other than x.

Thanks for your reply. It makes me realize that I wasn't specific enough in my question. Sorry about that. Let me clarify.

I have an input box that is used to define a function f in terms of x. I then want to display (as text) the function f'(x) evaluated at c (i.e. f'(c)). I can't seem to make your solution apply in this case.

Sorry again for the unclear question. I hope this is better now.

To show f'(c) in a text, reffering to f as well to c, create a text box, select in the list of objects the number c. Now a box with c is created. In this box you can change the content of the box from c into f'(c). So you can type, erase etc. in these textboxes, introducing formulas.

Thank you again, but again it seems I've been unclear. I'm confident this time my question will finally come through.

In the c_text.ggb file you provided, I want the text to read "2c" instead of "2" as it currently reads. In other words, I don't want to evaluate the function f'(x) at a number for c, I want to evaluate f'(x) at the variable c (replace all x's with c's).

Hi

My idea (…) is CAS with substitute command

Then you drag 2c (in the exemple) into graphics, and you have your text