How do I left and right justify text within an align environment

I want to simultaneously have text left and right justified around an align environment. I'm able to do this, but my solution seems like a dreadful hack. I really don't want to prepend every line with &&. But && doesn't really get the text to be flush with the left. Does anyone have a better solution?

Here's my code:

\documentclass{article}
\usepackage{amsmath,amssymb,amsthm,mathtools}
\usepackage[margin=1in]{geometry}
\setlength{\parindent}{0pt}
\begin{document}
Establish the following identity:
$\cos^4\theta-\sin^4\theta=\cos(2\theta)$
\begin{align*}%'
\tag*{\makebox[0pt][l]{\hspace*{-\textwidth}\textbf{Proof:}}}
\cos^4\theta-\sin^4\theta
&=     (1-\sin^2\theta)^2 - \sin^4\theta
&& \text{Use $\cos^2\theta=1-\sin^2\theta$}
\\
&    =    1 - 2\sin^2\theta + \sin^4\theta - \sin^4\theta
\\
&    =    1 - 2\sin^2\theta = \cos(2\theta) && \text{Double angle formula.}
\end{align*}
\end{document}


Result:

-
Welcome to tex.sx! Note that if you indent your code by 4 spaces, it will be displayed as a code block. Alternatively, you can select the code and click the "code" ({}) button. –  T. Verron Nov 22 '12 at 0:30
Welcome to TeX.SX! What is preventing you from writing "Proof" outside the align* environment? Having it on the same height as the first formula is not at all needed or desirable, in my opinion. –  egreg Nov 22 '12 at 0:33
It's just an example. It's not the best example. I agree with your comment about "proof". The point is that this example achieves the effect I desire. I'm interested in a better way of achieving the effect. –  A.Ellett Nov 22 '12 at 0:35
Left-aligned text "inside" align* is achieved using \intertext. See inserting sentences between subequations. –  Werner Nov 22 '12 at 0:41
flalign* really flushes left and right, but it still needs && hacks, as you said. –  Manuel Nov 22 '12 at 0:50

1 Answer

As in one of the comments mentioned I would use the not well known falign or falign*environment like in this example:

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

\begin{flalign}
\text{Proof}    && a &= b &&\\
&& a &= b &&\\
&& a &= b && \text{some comment}\\
&& a &= b &&
\end{flalign}

\begin{flalign*}
\text{Proof}    && a &= b &&\\
&& a &= b &&\\
&& a &= b && \text{some comment}\\
&& a &= b &&
\end{flalign*}

\end{document}


This will result in:

-
Is the only difference between the * and non-star versions the numbering? –  soandos Oct 3 '13 at 16:22
@soandos: As far as I know: Yes. –  Manuel Kuehner Oct 3 '13 at 17:22