# Math system with external parameter

I'm writing a document where I need to define a system with an external parameter. Here an image to make it clearer:

I'm using this current code to generate a common system, here is an example:

\documentclass{report}
\usepackage{mathtools}
\begin{document}

$$\begin{dcases} \mathbf{\dot r} \,= \frac{dr}{dt} \;= \mathbf{v} \\ \mathbf{\dot v} \,= \frac{dv}{dt} \;= g + \frac{\mathbf{T}}{m} \;= g + \mathbf{a_T}\\ \dot m = \frac{dm}{dt} = -\frac{T}{g_o I_\mathrm{sp}} = -\frac{T}{c} \end{dcases}$$

\end{document}


How could I rework the code to make an external parameter appear (likewise the psi greek letter along with the equality sign)?

Thank you!

• Is what you want achieved by adding \psi = immediately before \begin{dcases}? Commented Apr 13, 2021 at 16:30
• Thank you very much @WillieWong, It's exactly what I was looking for. It was much simpler than I thought. Commented Apr 13, 2021 at 16:34

I wouldn't say that a dcases environment is particularly well suited for the job at hand. I think you're better off with an alignedat environment, especially as it allows direct alignment of the = symbols.

\documentclass{report}
\usepackage{mathtools}
\begin{document}

\psi= \left\{ \begin{alignedat}{3} \dot{\mathbf{r}} &= \frac{dr}{dt} &&= \mathbf{v} \\ \dot{\mathbf{v}} &= \frac{dv}{dt} &&= g + \frac{\mathbf{T}}{m} &&= g + \mathbf{a_T}\\ \dot{m} &= \frac{dm}{dt} &&= -\frac{T}{g_o I_\mathrm{sp}} &&= -\frac{T}{c} \end{alignedat} \right.