If two triangles ABC and abc are oriented so that lines Aa, Bb, and Cc meet at a point, then the pairs of corresponding sides (AB and ab; BC and bc; and AC and ac) will meet in three collinear points.
If you want a proof, do a Google search on
"Proof of Desargues theorem".
Posted by Bractals
on 2016-10-30 04:53:00