Hi everyone!

Maybe the question sounds a little basic, but what I want to do is to get the derivative of a variable (say f) which is not defined a as function or expression, but, for my problem, it depends on time.

I need to calculate the derivative of an expression respect to the time, and so I want the variables to be in the form of f', f'' and so on.

When I use the d(f,t) command, the result is zero, because it doesn't depend on time explicitly.

Does anybody know a method to do this?

Cheers

Charlie

If the function f is not in terms of t, then a result of f' = 0 is indeed correct.

Yeah, I know. It's actually correct. But I need like a generic or symbolic derivative. I need to derive an expression which contains the f variable, and I want to get f' as a result of the derivative, where f' represents the generic derivative of f respect to the time.

Like motion equations, x is the position, x' is the speed, x'' is the acceleration and so on.

Hope you guys understand.

Thanks in advance

Charlie

Yeah, but the motion equation is:

x(t) = x0 + v0t +1/2at^2

It has t in it. You differentiate it with respect to t, and you get:

v(t) = x'(t) = v0 + at

Do it again, and you get:

a(t) = x''(t) = a

However, if you differentiated x(t) with respect to mass, you'd get dx(t)/dm = 0, as you should, since that's meaningless.

The expression that I have to derive respect to time is the Lagrangian (I'm currently working for the Robotics tools for TI-89), which is:

d/dt [ ∂L/∂ q˙i - ∂L/∂qi = Qi ]

Note the q˙i is the first derivative of qi, which I don't know, but it depends on time. So, when I get derivative I'd like the result containing the q˙i and q˙˙i and so on.

I hope you guys understand.

Thanks for your answers.

Yeah, I see what you're trying to do; I dealt with all this fun stuff in my EE courses back in the day.

I'm pretty sure that the TI-89 isn't capable of taking that sort of a derivative, but hopefully some 68k user can better clarify.

**KermMartian wrote:**

Yeah, I see what you're trying to do; I dealt with all this fun stuff in my EE courses back in the day.

I'm pretty sure that the TI-89 isn't capable of taking that sort of a derivative, but hopefully some 68k user can better clarify.

Unfortunately, this is required for my TI-Basic Robotics project. Specifically, for the last part.

Thanks for your answers Kerm
I found the solution. The TI89 Titanium can apply the chain rule correctly, but the result it gives isnt like f'. It will present the result as d/dt f(t).

In fact, I did:

**Code:** `d((f(t))^2,t) `

and the result is:

**Code:** `2f(t)*d/dt f(t)`

The d/dt f(t) in the result is actually f'. So, everything was just about notation, which makes me feel happy, because I found the solution for one of the problems of my robotics project.

Cheers!
Glad to hear you got it figured out.

I don't know nearly as much about using the 89 for math as I'd like, so it's neat to see that it can do this sort of thing.

Charlie, I appreciate you updating this topic with the correct solution for the sake of future visitors searching for the answer to their woes. And I'm glad it solved a tangible problem (other than the math itself) for you!