The figure shows the triangles ABC and
A'B'C' so that AA', BB', CC' meet at a point O. AB and A'B' meet
at D, AC and A'C' at E, BC and B'C' at F. Prove that D,E, and F are on a
line (collinear points).
Note. The triangles ABC and A'B'C' are said to be in perspective, O is
called the center of perspective or perspector. Line DEF is called the
axis of perspective or perspectrix.