How may I write 'and generally' and the second equation on one line?

Question

Please note that

1. the two equations ought to be aligned;
2. the text and generally should be flush left, but the second equation should be centred.

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{flalign*}
  a^{m}\times a^{n}&=
  a^{m+n},\\
  \intertext{and generally}
  a^{m}\times a^{n}\times a^{p}\times \dotsm&=
  a^{m+n+p+\dotsb}
\end{flalign*}
\end{document}

Answer 1

Here is an option that sets both equations in a \linewidth box, measuring the duplicating the widest parts in the shorter (first) equation in order to center the contents. It also allows the placement of and generally to be flush-left:

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

\begin{align*}
  a^m \times a^n &= a^{m + n}, \\
  \intertext{and generally}
  a^m \times a^n \times a^p \times \dotsm &= a^{m + n + p + \dotsb}
\end{align*}

\begin{align*}
  \makebox[\linewidth]{%
    $\hphantom{a^m \times a^n \times a^p \times \dotsm}
     \mathllap{a^m \times a^n} = 
     \mathrlap{a^{m + n},}
     \hphantom{a^{m + n + p + \dotsb}}$} \\
  \makebox[\linewidth]{%
    \rlap{and generally}\hfill%
    $a^m \times a^n \times a^p \times \dotsm = a^{m + n + p + \dotsb}$%
    \hfill}
\end{align*}

\end{document}

Overlapping macros allow for the setting and placement of content without it taking up any horizontal space.

Answer 2

Use flalign (full length align). However, in my opinion \intertext or \shortintertext are better: I find no reason that justifies “and generally” being on the same line as the equation.

\documentclass{article}
\usepackage{mathtools}
\usepackage{lipsum} % just for the example

\begin{document}

\lipsum*[2]
\begin{align*}
a^m \times a^n &= a^{m + n}, \\
\shortintertext{and generally}
a^m \times a^n \times a^p \times \dotsm &= a^{m + n + p + \dotsb}
\end{align*}
\lipsum*[2]
\begin{flalign*}
&& a^m \times a^n &= a^{m + n}, & \\
\makebox[0pt][l]{and generally}&&
a^m \times a^n \times a^p \times \dotsm &= a^{m + n + p + \dotsb} &
\end{flalign*}
\lipsum[2]

\end{document}