[Tex/LaTex] Tagging multi line equations

Question

Often I have a chain of equations but later on I only want to refer that the first statement is equal to the last one. There are some options to do that:

\begin{align}
  \label{equation}
      a    
  &=  b \\ 
  &=  c \\
  &=  d \\
  &=  e \\
  &=  f
\end{align}
Equation~\eqref{equation} gives $a=f$.

\begin{align}
      a    \label{equation_first}
  &=  b \\ 
  &=  c \\
  &=  d \\
  &=  e \\
  &=  f    \label{equation_last}
\end{align}
Equation~\eqref{equation_first}--\eqref{equation_last} gives $a=f$.

\newcommand\numberthis{\addtocounter{equation}{1}\tag{\theequation}}
\begin{align*}
      a    
  &=  b \\ 
  &=  c \\
  &=  d \\
  &=  e \\
  &=  f    \numberthis \label{equation2}
\end{align*}
Equation~\eqref{equation2} gives $a=f$.

\begin{equation}
  \label{equation_split}
  \begin{split}
        a    
    &=  b \\ 
    &=  c \\
    &=  d \\
    &=  e \\
    &=  f 
  \end{split}
\end{equation}
Equation~\eqref{equation_split} gives $a=f$.

• The first one is just bad, as (2)–(5) are displayed, but not included in the reference.
• The second one is more or less okay, but after all it is one equation while it looks like I need six equations.
• The third one is a hack I found here (alternativly I could have used align* and lots of nonumbers). This version is okay, but does not look that good. Sure, I could put the tag on the center line, but that‘s manual work (I have to (not forget to) redo that stuff after adding some lines) and there is not always a center line.
• The fourth option looks quite good. However, I pretty much always used align[*] up to now. Are there any problems with equation+split?

How do you tag multi line equations? Why?

Answer 1

You could use the aligned environment:

\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{hyperref}
\begin{document}
\begin{equation}
    \begin{aligned}
    a &= b\\
      &= c\\
      &= d\\
      &= e\\
      &= f
      \label{eq:EqAligned}
    \end{aligned}
\end{equation}
Equation \ref{eq:EqAligned}...
\end{document}

It centers the number vertically even if the number of equations is even, and the reference works fine.