\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{alignat*}{2}
&\qquad&
  \frac{\lvert\overline{AD}\rvert}{\lvert\overline{CD}\rvert}
    &=\frac{\lvert\overline{DC}\rvert}{\lvert\overline{CE'}\rvert}\\
\iff&& \frac{1}{x}&=\frac{x}{1-x}\\
\iff&& 1-x&=x^2\\
\iff&& 0&=x^2+x-1
\end{alignat*}

\end{document}

\starttext
\startformula
  \startmathalignment[n=3]
   \NC \qquad \NC\frac{\lvert\overline{AD}\rvert}{\lvert\overline{CD}\rvert} \NC =\frac{\lvert\overline{DC}\rvert}{\lvert\overline{CE'}\rvert}\NR
   \NC \iff \NC  \frac{1}{x}  \NC =\frac{x}{1-x} \NR
   \NC \iff \NC 1-x \NC =x² \NR
   \NC \iff \NC 0\NC =x²+x-1 \NR
  \stopmathalignment
\stopformula
\stoptext