Definition a_ratio A O U V ro ru rv := 
 ~ Col O U V /\ 
 S A U V / S O U V = ro /\
 S O A V / S O U V = ru /\
 S O U A / S O U V = rv.Definition is_centroid G A B C :=  a_ratio G A B C (1/(2+1)) (1/(2+1)) (1/(2+1)).Definition is_orthocenter' H A B C := 
  a_ratio H A B C (Py A B C * Py A C B / ((2*2*2*2) * (S A B C * S A B C)))
                            (Py B A C * Py B C A / ((2*2*2*2) * (S A B C * S A B C)))
                            (Py C A B * Py C B A / ((2*2*2*2) * (S A B C * S A B C))).Definition is_orthocenter H A B C := 
 a_ratio H A B C 
        (Py A B C * Py A C B / (Py A B A * Py A C A - Py B A C * Py B A C))
        (Py B A C * Py B C A / (Py A B A * Py A C A - Py B A C * Py B A C))
        (Py C A B * Py C B A / (Py A B A * Py A C A - Py B A C * Py B A C)).

Lemma is_orthocenter_non_zero : forall H A B C, is_orthocenter H A B C -> (Py A B A * Py A C A - Py B A C * Py B A C) <> 0.