LATEX Mini HOWTO  
a=\left[\frac{\partial F}{\partial x}+\frac{\partial G}{\partial y}\right]
\dot{m} \overline{a+b} \tilde{a} a^\prime
\begin{equation} F=ma \tag{23a} \label {eqn:newton} \end{equation} Eq. (\ref{eqn:newton}) 




$\pi$ 