Menelaus' Theorem
Statement
(awaiting image) A necessary and sufficient condition for points D, E, F on the respective side lines BC, CA, AB of a triangle ABC to be collinear is that
![$BD*CE*AF = -DC*EA*FB$](http://latex.artofproblemsolving.com/1/3/9/139c610543de676b7d26ca606f8bf4ed6a9b87d1.png)
where all segments in the formula are directed segments.
Example
Does anyone have a good example?