Text at end of math-line in align environment [duplicate]

Question

How to put text at the end of a line (outmost right) in an align* environment? I have a set of equations in align* and at the end of each line I want to write a little bit text which explains that line. And I want it to be at the right of each line.

Answer 1

There are two different kinds of objects that can adorn an equation:

1. a number or tag or
2. an “explanation”.

The number or tag doesn't belong in the equation: it's just a convenience for cross references, handier than “the third equation on page 234”.

An explanation instead is part of the equation (or the step in a derivation) and should not be flush right. The code:

\begin{alignat*}{2}
a+(b+s(c))
&=a+s(b+c)    &\qquad& \text{(definition)} \\
&=s(a+(b+c))  && \text{(definition)} \\
&=s((a+b)+c)) && \text{(induction)} \\
&=(a+b)+s(c)  && \text{(definition)}
\end{alignat*}

The same (ab)using \tag:

\begin{align*}
a+(b+s(c))
&=a+s(b+c)    \tag{definition} \\
&=s(a+(b+c))  \tag{definition} \\
&=s((a+b)+c)) \tag{induction} \\
&=(a+b)+s(c)  \tag{definition}
\end{align*}

But this has an obvious problem: if the document class specifies leqno, you'd get

with no warning.

A possible way out is to use flalign*

\documentclass[leqno]{article}
\usepackage{amsmath}
\usepackage{showframe}

\begin{document}

After observing that $a+(b+0)=a+b=(a+b)+0$ we can proceed to prove
the associativity of addition by induction, namely
\begin{flalign*}
&&a+(b+s(c))
&=a+s(b+c)    &&\text{(definition)} \\
&&&=s(a+(b+c))  &&\text{(definition)} \\
&&&=s((a+b)+c)) &&\text{(induction)} \\
&&&=(a+b)+s(c)  &&\text{(definition)}
\end{flalign*}


\end{document}

But I'd stay with the first display, for the reasons outlined at the beginning.