How do I add a line break in display math mode?

Question

For instance, how do I add a break after the first line in

\[
    a + b = c // what should I add here?
    a = c - b
\]

Answer 1

The \[...\] is used to typeset a single equation, not multiple equations.

You need to use an environment that allows for multiple equations. One way is to use gather, but I normally use align, both from the amsmath package:

Notes:

• With display math environments if you leave blank lines in between you get additional vertical spacing and the possibility of a page break between a paragraph and following display.
• If you don't want the equations numbered use the starred versions of the environments: gather* and align*.
• A really good reference is Herbert Voss's comprehensive review of mathematics in (La)TeX.

Code:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{gather}
    a + b = c \\ 
    a = c - b
\end{gather}
%
With the align environment you can align equations:
%
\begin{align}
    a + b &= c \\
    a &= c - b
\end{align}
\end{document}