MuPAD equation solving

Question

I have x(t) defined as:

x(t) = Symbol::subScript(x,0) + Rb*`φ`(t)

I want to write the derivative of phi(t) in terms of x(t). When I try

diff(`φ`(t),t)

it doesn't substitute from the x(t) equation.

Answer 1

Note, that for MuPAD assignments only happens when you use a:=1 and not with a=1.

So you should write

x(t) := Symbol::subScript(x,0) + Rb*`φ`(t)

instead. But then

diff(`φ`(t),t) 

only gives \begin{align} \frac{\partial}{\partial t} \phi(t)\end{align}

However, it works the other way around

`φ`(t) := 1/Rb*(x(t)-Symbol::subScript(x,0))

Then you can use

diff(`φ`(t),t) 

to get \begin{align} \frac{\frac{\partial}{\partial t} x(t)}{Rb} \end{align}