equations
I have four equations, and I'd like to display them in two rows and two columns. I also want them to have their own respective equation numbering. I've tried
\begin{multicols}{2}
\begin{equation}
A=B
\end{equation}
\columnbreak
\begin{equation}
C=D
\end{equation}
\end{multicols}
But if I do that, they aren't aligned. Could anyone point me in the right direction?
Use the alignedat environment:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{alignedat}{2}
\textup{[M/H]} &\cong 0.917 \textup{[Fe/H]} &&(1)\\
&\cong 0.977 \textup{[Fe/H]} &&(2)\\
&\cong \textup{[Fe/H]} + \log(0.638f_{\alpha}+0.362) \quad&&(3)
\end{alignedat}
\end{equation}
\end{document}
The \; commands you're using are wrong, as they introduce an asymmetric space. Also the chemical symbols should be upright.
If that is what you had in mind, here is the code:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align} \label{eq:ComptonWavelength}
\Delta\lambda &= \frac{h}{m_e c^2}(1 - \cos{\theta)}
\intertext{or}
\theta &= \arccos{\biggl(1 + m_e c^2 \biggl(\frac{1}{E_\gamma} - \frac{1}{E'_\gamma} \biggr) \biggr)}
\end{align}
\end{document}
If you want the vertical spacing around the "or" to be less, you can use \shortintertext instead of \intertext, but then you need the package mathtools instead of amsmath.
To place the equations side by side (I do not recommend this), you can try two minipages with [t] alignment:
But you'll need a \vphantom{\bigg(} so that the tops line up.
Here is the code:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{minipage}[t]{.35\textwidth}
\begin{equation} \label{eq:ComptonWavelength}
\vphantom{\bigg(}\Delta\lambda = \frac{h}{m_e c^2}(1 - \cos{\theta)}
\end{equation}
\end{minipage}
\begin{minipage}[t]{.6\textwidth}
\begin{equation}
\text{or\quad} \theta = \arccos{\biggl(1 + m_e c^2 \biggl(\frac{1}{E_\gamma} - \frac{1}{E'_\gamma} \biggr) \biggr)}
\end{equation}
\end{minipage}
\end{document}
You'll have to play around with the widths of the minipages. But I think it looks cramped all on one line.
Best Answer
If the equations in the left- and right-hand columns have (roughly) equal heights, it suffices to insert the instruction
after
begin{multicols}{2}.