# [pstricks] tangent to \psellipse or \pscircle

Patrick Drechsler patrick at pdrechsler.de
Tue Oct 30 02:53:33 CET 2007

Patrick Drechsler <patrick at pdrechsler.de> writes:

> what is the best way to draw a tangent to \psellipse or \pscircle at
> certain coordinates? And how does one retreive these coordinates?

Using pst-eucl solved the problem at hand.

For the record:

--8<---------------cut here---------------start------------->8---
\documentclass{article}
\usepackage{pstricks}
\usepackage{pst-eucl}
\SpecialCoor%
\begin{document}
\begin{pspicture}[showgrid=true](-3,-3)(2.5,2)
%% Circle:
\pstGeonode(0,0){O}%
\pstGeonode(2,0){A}%
\pstCircleOA{O}{A}% draw circle
%% Line:
\pstGeonode[PosAngle=135](-2,-1.5){W1}%
\pstGeonode[PosAngle=135](3,-1.5){W2}%
\psline[linewidth=0.5pt,linecolor=gray](W1)(W2)% draw line
%% Intersection between line and circle:
\pstInterLC[PosAngleA=-90,PosAngleB=-45]{W1}{W2}{O}{A}{SP1}{SP2}
%% Secant between the above intersection points SP1 and SP2:
\psline[linecolor=blue](SP1)(SP2)
\psline[linecolor=cyan](O)(SP1)
%% Tangent:
\psRelLine[linecolor=red,angle=90](SP1)(O){0.5}{EndnodeA}
\psRelLine[linecolor=red,angle=-90](SP1)(O){0.5}{EndnodeB}
\rput([offset=-.3]EndnodeB){{\color{red}tangent}}
\end{pspicture}
\end{document}
--8<---------------cut here---------------end--------------->8---

There are probably more elegant ways of doing this.

Cheers,

Patrick
--
Statistics are like bikinis. What they reveal is suggestive, but what
they conceal is vital. --Aaron Levenstein