Typesetting multi-line formulas

We shall use the amsmath package to typeset a very long formula and a system of equations:

1. Start a new document on A6 paper size to have a smaller text width, so we can see what happens with line breaks without typing super long formulas:

   \documentclass{article}
   \usepackage[a6paper]{geometry}

2. Load the amsmath package, and begin the document:

   \usepackage{amsmath}
   \begin{document}

3. Use the multline environment to span a long equation over three lines. End the lines with a double backslash \\, except the last one:

   \begin{multline}
     \sum = a + b + c + d + e \\
              + f + g + h + i + j \\
              + k + l + m + n
   \end{multline}
   \end{document}

4. Click on Typeset to compile, and look at the formula:

   Figure 9.16 – A formula spanning three lines

5. Now we handle a system of equations. Use the gather environment...