Help with Derivatives in Mathcad

[czechman45]

I know that Mathcad only takes partial derivatives. I set up my equations using this general format:

L:= x(t)

Then, I take the derivative of L with respect to t and get the following:

dL/dt -> d/dt*x(t)

However, when I take the derivative of L with respect to x, I should get 1, but am getting 0.

dL/dx -> 0

I've also tried typing dL/d(x(t)), but the program gets mad at me saying that I'm not using a name. Please help. I need Mathcad to take the derivatives of some big expressions with respect to both time and specific variables.

Thank you.

 

[I like Serena]

Hi czechman45, welcome to PF

I know that Mathcad only takes partial derivatives. I set up my equations using this general format:

L:= x(t)

Then, I take the derivative of L with respect to t and get the following:

dL/dt -> d/dt*x(t)

However, when I take the derivative of L with respect to x, I should get 1, but am getting 0.

dL/dx -> 0

I've also tried typing dL/d(x(t)), but the program gets mad at me saying that I'm not using a name. Please help. I need Mathcad to take the derivatives of some big expressions with respect to both time and specific variables.

Thank you.

Since you have defined L without a parameter, as far as Mathcad knows, L is not dependent on any parameter, meaning it is treated as a constant.

To fix it you need to define:
L(t) := x(t)

This explains to Mathcad that L is a function with a parameter, and that x(t) is supposed to be used with this parameter.

 

[czechman45]

@I like Serena
I tried your suggestion. It works for dL/dt, but it still gives me 0 for dL/dx, where I'm supposed to be getting 1. Any other suggestions?

 

[I like Serena]

@I like Serena
I tried your suggestion. It works for dL/dt, but it still gives me 0 for dL/dx, where I'm supposed to be getting 1. Any other suggestions?

I'm guessing a bit, since I don't know what your large expressions look like.

I suspect you need L to be of the form:

L(t,x) := x

Now you can take d/dt from L(t,x(t)) and you can take d/dx from L(t,x).
The first derivative would be a total derivative, while the second is a partial derivative.

In particular each derivative you want to take requires L to have a parameter with the same name.