Align equations with equivalent and equal sign

I am currently trying to get the align environment to work as I want it to work.

\begin{align}
&& a \cdot c + b &= 0\\
\Leftrightarrow && a \cdot c &= -b\\
\end{align}

This code will align the equivalent sign in the middle of the left half and the equal sign in the middle of the right half. What I actually want is that the equivalent sign is on the outer left of the page and the formula is in the middle of the remaining page. How can this be done and is this even encouraged? Or is there a better way to align formula with equivalent and equal signs?

Here are two options: \documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
a \cdot c + b &= 0 \\
\Leftrightarrow \qquad a \cdot c &= -b
\end{align}

\begin{align}
a \cdot c + b &= 0 \\
\llap{$\Leftrightarrow$ \qquad} a \cdot c &= -b
\end{align}

\begin{align}
a \cdot c + b &= 0 \\
a \cdot c &= -b \refstepcounter{equation}\tag*{(\theequation)\llap{\makebox[\linewidth][l]{$\Leftrightarrow$}}}
\end{align}
\end{document}

The first sets the \Leftrightarrow a distance of \qquad from the second equation. The second does the same, but removes any influence of \Leftrightarrow in terms of the horizontal spacing. The third sets it as part of the \tag* in order to position it on the left of the text block boundary.

The text block boundary is obtained by using the showframe package; not included in the above MWE.

• I like both, thx! – Kamairo Jul 11 '14 at 18:14
• The first two are ambiguous, the third is abominable. ;-) – egreg Jul 11 '14 at 20:54

The first construction I propose below is sometimes used in a blackboard; for a written document, I'd suggest using the natural language and \shortintertext from mathtools:

\documentclass[11pt]{article}
\usepackage{mathtools}
\usepackage{graphicx}

\begin{document}

\begin{align}
a \cdot c + b &= 0 \\
&\rotatebox[origin=c]{-90}{$\Leftrightarrow$} \notag \\
a \cdot c &= -b
\end{align}

\begin{align}
a \cdot c + b &= 0 \\
\shortintertext{if and only if}
a \cdot c &= -b
\end{align}

\end{document} • Definitly options if theres only 2 or 3 formula. – Kamairo Jul 11 '14 at 18:14
• @Kamairo it's nor to demerit Werner's answer (he gives what you asked for), but take into account that in all my years as a mathematician I've never ever seen that way to represent the equivalence between two expressions, specially the one with the sign flushed to the margin. – Gonzalo Medina Jul 11 '14 at 18:18
• Or precede the equations by “the two following equations are equivalent”; I agree with the idea of avoiding symbols. – egreg Jul 11 '14 at 20:55