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How can I align this equation to the left?

\begin{equation}
\beta_1=(n_{20}+n_{02})
\\
\beta_2={(n_{20}-n_{02})^2}+4{n_{11}}^2
\\
\beta_3={(n_{30}-3n_{12})^2}+{(3n_{21}-n_{03})^2}
\\
\beta_4={(n_{30}+n_{12})^2}+{(n_{21}-n_{03})^2}
\\
\beta_5=(n_{30}-3n_{12})(n_{30}+n_{12})[{(n_{30}+n_{12})^2}-3{(n_{21}+n_{03})^2}]+(3n_{21}-n_{03})(n_{21}+n_{03})[3{(n_{30}+n_{12})^2}-{(n_{21}+n_{03})^2}
\\
\beta_6=(n_{20}-n_{02})[{(n_{30}+n_{12})^2}-{(n_{21}+n_{03})^2}]+4n_{11}(n_{30}+n_{12})(n_{21}+n_{03})
\\
\beta_7=(3n_{21}-n_{03})(n_{30}+n_{12})[{(n_{30}+n_{12})^2}-3{(n_{21}+n_{03})^2}]-(n_{30}+3n_{12})(n_{21}+n_{03})[3{(n_{30}+n_{12})^2}-{(n_{21}+n_{03})^2}]
\label{Eq:momentsHU}
\end{equation}

marked as duplicate by Community Feb 24 '16 at 21:14

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2

It can't work: equation is for one-lined equations. Use aligned from amsmath, are aligned inside equation, if you want only one equation number. The longest equations will have to be split again, which you can do, say, with multlined:

\documentclass[a4paper, 11pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[showframe]{geometry}
\usepackage{mathtools}

\begin{document}

\begin{equation}
  \begin{aligned}
    \beta_1 & =(n_{20}+n_{02})
    \\
    \beta_2 & ={(n_{20}-n_{02})^2}+4{n_{11}}^2
    \\
    \beta_3 & ={(n_{30}-3n_{12})^2}+{(3n_{21}-n_{03})^2}
    \\
    \beta_4 & ={(n_{30}+n_{12})^2}+{(n_{21}-n_{03})^2}
    \\
    \beta_5 & =\!\begin{multlined}[t]
    (n_{30}-3n_{12})(n_{30}+n_{12})[{(n_{30}+n_{12})^2}-3{(n_{21}+n_{03})^2}]
    \\+(3n_{21}-n_{03})(n_{21}+n_{03})[3{(n_{30}+n_{12})^2}-{(n_{21}+n_{03})^2}
    \end{multlined}
    \\
    \beta_6 & =(n_{20}-n_{02})[{(n_{30}+n_{12})^2}-{(n_{21}+n_{03})^2}]+4n_{11}(n_{30}+n_{12})(n_{21}+n_{03})
    \\
    \beta_7 & =\!\begin{multlined}[t]
    (3n_{21}-n_{03})(n_{30}+n_{12})[{(n_{30}+n_{12})^2}-3{(n_{21}+n_{03})^2}]\\ -(n_{30}+3n_{12})(n_{21}+n_{03})[3{(n_{30}+n_{12})^2}-{(n_{21}+n_{03})^2}]
    \end{multlined}
    \label{Eq:momentsHU}
  \end{aligned}
\end{equation}

\end{document} 

enter image description here

2

I would just set it using align* (include \usepackage{amsmath} in your preamble) and break the lengthy lines at appropriate locations. You may consider using flalign* as well:

enter image description here

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{align*}
  \beta_1 &= (n_{20} + n_{02}) \\
  \beta_2 &= (n_{20} - n_{02})^2 + 4 n_{11}^2 \\
  \beta_3 &= (n_{30} - 3n_{12})^2 + (3n_{21} - n_{03})^2 \\
  \beta_4 &= (n_{30} + n_{12})^2 + (n_{21} - n_{03})^2 \\
  \beta_5 &= (n_{30} - 3n_{12})(n_{30} + n_{12})[(n_{30} + n_{12})^2 - 3(n_{21} + n_{03})^2] + {} \\
          &\phantom{{}={}} \qquad (3n_{21} - n_{03})(n_{21} + n_{03})[3(n_{30} + n_{12})^2 - (n_{21} + n_{03})^2 \\
  \beta_6 &= (n_{20} - n_{02})[(n_{30} + n_{12})^2 - (n_{21} + n_{03})^2] + 4n_{11}(n_{30} + n_{12})(n_{21} + n_{03}) \\
  \beta_7 &= (3n_{21} - n_{03})(n_{30} + n_{12})[(n_{30} + n_{12})^2 - 3(n_{21} + n_{03})^2] - {} \\
          &\phantom{{}={}} \qquad (n_{30} + 3n_{12})(n_{21} + n_{03})[3(n_{30} + n_{12})^2 - (n_{21} + n_{03})^2]
\end{align*}

\end{document}
1

This is more or less the same question as Align equation left. You can use the flalign environment or load your documentclass with the fleqn option.

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