Proof with Geometry.
intros...
assert (S R A B / S Q A B = S R B A / S Q B A)...
rewrite H2...
Qed.
Hint Resolve A6_6: Geom.

Theorem l2_7 :
forall A B C D P : Point,
Col C A B ->
Col D A B ->
~ Col P C A -> ~ Col P A B -> A <> B -> S P C D / S P A B = C ** D / A ** B.