Aligning multiple lines of text in an align environment

Question

I want to use the right side of an aligned environment to present the reason for the next line. If the explanation requires more than one line the alignment fails. This could be eye-balled by adding a horizontal space but there should be a better way.

How can this be fixed?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
  a + b &= \quad \text{short reason} \\
  c + d &= \quad \text{a really long and} \\
        &\phantom{=} \quad \text{complicated reason} \notag \\
  e + f \text{.}
\end{align}
\end{document}

If you compile the above code you will see what I mean. I can't figure out how to add compiled code to a question. If you know how do add compiled code to the question, please do so.

Answer 1

One option is to use the aligned construct for the multi-line comments as below.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
  a + b &= \quad \text{short reason} \\
  c + d &= \quad \begin{aligned}[t]&\text{a really long and} \\
                                   &\text{complicated reason}\end{aligned}\\
  e + f \text{.}
\end{align}
\end{document}

Answer 2

with use of parbox:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
  a + b &= \quad \text{short reason} \\
  c + d &= \quad \parbox[t]{0.35\textwidth}{
                    really long and complicated\\
                    reason in two lines} \\
  e + f &=\text{.}
\end{align}
\end{document}

(red lines indicate text borders)

supplement: considering comment of barbara beton, the above solution should be:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
  a + b & = \quad \text{short reason} \\
  c + d & = \quad \parbox[t]{0.35\textwidth}{\raggedright  % <--- 
                    really long and complicated\\
                    reason in two lines} \\
  e + f & = .
\end{align}
\end{document}

in this simple case result is the same as at the first case, however if there's really long word that can't fit on the first line, added \ragged will avoid wide spaces between words in the first line.

Answer 3

A variation on @AboAmmar's answer, but using an environment that takes text and paragraphs:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
  a + b &= \quad \text{short reason} \\
  c + d &= \quad \begin{tabular}{@{}p{0.8\textwidth}}
                    really long and\\
                    complicated reason
                 \end{tabular} \\
  e + f &=\text{.}
\end{align}
\end{document}

to produce

Of course, the 0.8\textwidth might need tweaking...

Answer 4

Another way could be using \begin{center} line_one\\line_two \end{center}

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{center}
      \begin{equation}
        a + b = \quad \text{short reason}
      \end{equation}  
      \begin{equation}
        c + d = \quad \parbox[t]{0.35\textwidth}{
                        really long and complicated\\
                        reason in two lines}
      \end{equation}
      \begin{equation}
        e + f =\text{.}
      \end{equation}
\end{center}
\end{document}

The result would be like below image (Green color is drawn by hand just for showing the page's border)