split a long, multiline formula over two lines

to split a long, multiline formula over two lines, import the "amsmath" package and use the "split command" and double slashes.

for instance, to produce the above equation, use the following code:

\usepackage{amsmath}
...

\begin{split}
\arg\max_\Theta\left[\sum_zP(\mathcal{G}|\mathcal{D},\mathcal{S})
P(\mathcal{G}|\mathcal{S},\Theta_n)\log{P(\mathcal{D}|\mathcal{G},
\mathcal{S},\Theta)}\right. \\
+
\left.\sum_zP(\mathcal{G}|\mathcal{D},\mathcal{S})P(\mathcal{G}|
\mathcal{S},\Theta_n)\log{P(\mathcal{D}|\mathcal{G},\mathcal{S},\Theta)}\right]
\end{split}