[Tex/LaTex] New line in Latex Equation

Question

  \begin{equation}
    S=\sum\limits_{i=1}^{8}A_i
    S=\sum\limits_{i=1}^{8}B_i
    S=\sum\limits_{i=1}^{8}C_i
    S=\sum\limits_{i=1}^{8}D_i
  \end{equation}

Considering the above code, I am so interested to know that how it is possible to print these 4 equations, one per line, instead of all followed in one single line.

Answer 1

Package amsmath provides many environments for equations, e.g.:

• gather for centering equations by default
• align, which allows vertical alignments

Example:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
  \begin{gather}
    S=\sum_{i=1}^{8}A_i\\
    S=\sum_{i=1}^{8}B_i\\
    S=\sum_{i=1}^{8}C_i\\
    S=\sum_{i=1}^{8}D_i
  \end{gather}
  \begin{align}
    S &= \sum_{i=1}^{8}A_i\\
    S &= \sum_{i=1}^{8}B_i\\
    S &= \sum_{i=1}^{8}C_i\\
    S &= \sum_{i=1}^{8}D_i
  \end{align}
\end{document}

Equation number in last line

\documentclass{article}
\usepackage{amsmath}

\begin{document}
  \begin{gather}
    \nonumber S=\sum_{i=1}^{8}A_i\\
    \nonumber S=\sum_{i=1}^{8}B_i\\
    \nonumber S=\sum_{i=1}^{8}C_i\\
    S=\sum_{i=1}^{8}D_i
  \end{gather}
  \begin{align}
    \nonumber S &= \sum_{i=1}^{8}A_i\\
    \nonumber S &= \sum_{i=1}^{8}B_i\\
    \nonumber S &= \sum_{i=1}^{8}C_i\\
    S &= \sum_{i=1}^{8}D_i
  \end{align}
\end{document}

Equation number in the middle

\documentclass{article}
\usepackage{amsmath}

\begin{document}
  \begin{equation}
  \begin{gathered}
    S=\sum_{i=1}^{8}A_i\\
    S=\sum_{i=1}^{8}B_i\\
    S=\sum_{i=1}^{8}C_i\\
    S=\sum_{i=1}^{8}D_i
  \end{gathered}
\end{equation}
\begin{equation}
  \begin{aligned}
    S &= \sum_{i=1}^{8}A_i\\
    S &= \sum_{i=1}^{8}B_i\\
    S &= \sum_{i=1}^{8}C_i\\
    S &= \sum_{i=1}^{8}D_i
  \end{aligned}
\end{equation}
\end{document}

Addendum: sign function definition with cases

\documentclass{article}
\usepackage{amsmath}
\usepackage{colonequals}

\DeclareMathOperator{\sgn}{sgn}

\begin{document}
  \begin{equation}
    \sgn(x) \colonequals
    \begin{cases}
      -1 & \text{if } x < 0 \\
      0  & \text{if } x = 0 \\
      1  & \text{if } x > 0
    \end{cases}
  \end{equation}
\end{document}