\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-eucl}
\addtopsstyle{gridstyle}{gridlabels=0pt,griddots=0}
\makeatletter
\begin{document}
\begin{pspicture}[showgrid](-2,-2)(2,2)
\pstGeonode[PosAngle={180,45,-90}]
(-1.5,-1){A}
(1.5,1){B}
(1.5,-1){C}
\pstRightAngle[RightAngleSize=0.2,fillstyle=solid,fillcolor=green!50]{A}{C}{B}
\pstVerb{%
tx@NodeDict begin
/N@A load GetCenter
/N@B load GetCenter
Pyth2 \pst@number\psrunit\space div
/Dist ED
end}%
\pnode[A](!Dist 21 PtoC){D}\psdots(D)