## Differentiate function by a function

Moritz

### Moritz (view profile)

님이 질문을 제출함. 19 Apr 2012
최근 활동 Manali Gupta

### Manali Gupta (view profile)

님이 편집함. 21 Jun 2019
Hi
I'm just a little bit lost here. I'm using the latest MATLAB release with the symbolic maths toolbox. At the moment I'm working on a system, which has equations like x=theta(t)+2 (of course a lot more complicated and longer). Now I would like to differentiate this equation by theta(t). Hence, I should get x=1. However, if I use the diff(x,theta) command I only get the message Invalid variable.
How do I do it? What am I doing wrong?
Thanks!

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## 답변 수: 1

Manali Gupta

### Manali Gupta (view profile)

님의 답변 21 Jun 2019
Manali Gupta

### Manali Gupta (view profile)

님이 편집함. 21 Jun 2019

Hi Moritz,
Here theta(t) is not a symbolic variable but a symbolic function. You can differentiate a symbolic expression only w.r.t to a symbolic variable and not a symbolic function. Please refer to the documentation for more details.

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