A ratio that is invariant under projection.
________
Desargues:
[b]→1) Pappus' Theorem[/b]
2) Vector notation: [url]http://www.geogebratube.org/material/show/id/35213[/url]
3) Desargues' Theorem (corresponding/similar figures... drawing perspective lines) [url]http://www.geogebratube.org/material/show/id/35525[/url]
