(* The line going through the midpoints is parallel to the third side *)

Theorem Conversemenelaus :
forall (A B C D E G : Point) (r1 r2 : F),
mratio D B C r1 ->
mratio E C A r2 ->
inter_ll G D E A B ->
G <> A ->
parallel B G G A ->
B ** G / G ** A = - r1 * r2.