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Help with symbolic derivatives in Mathcad

czechman45
1-Newbie

Help with symbolic derivatives in Mathcad

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

L:= f(x(t),y(t)) // Note: I do not mean here that


for example L:= 5*x(t) ^2 + 5*y(t)

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

dL/dt -> 10*x(t)*d/dt(x(t)) + 5*y(t)*d/dt(y(t))

However, when I take the derivative of L with respect to x, I should get 10*x(t), 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. These specific variables are dependant on time, but there is no expression for them. By that I mean that x:= x(t), but there is no expression for x(t).
Also, is there a way to differentiate a function with respect to a derivative of one of the variables? For example: take the symbolic derivative of dL/dt with respect to d/dt(x(t)).

Thank you.
Also, if you know the answers to any of these other questions, feel free to post answers.

Can you use Greek letters as variable names and can variable names contain subscripts?

Can you make an equation wrap so that it doesn't go off of the printable page?

Is there a way to define symbolic constants? Say, 'm1' to represent mass one. I mean, is there a way to let Mathcad know that even though it is a symbol it is a constant?

How to solve a system of n equations of n variables in Mathcad?

Does Mathcad have some sort of ode solver (I am familiar with ode45 in Matlab)?

Can it do animations (plots over time) ?

1 REPLY 1

Daniel

I put some information in the attached file, but basically:

1) You are asking for derivatives of function with respect to another function; However this is variational calculus, not differential calculus, and I don't think that is what you want, see the file for more.

2) Much of what you are asking is easily found in help

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