(* step 2 *)
(* Intersection between the unit circle and D_r *)
Definition interCDr (r x y : R) := (in_ucirc x y) /\ (D_r r x y).
(* Points of the intersection *)
Definition p1 := (-1,0)%R.
Definition p2 (r : R) :=
let den := (1 + r * r)%R in
((1 - r * r) / den, 2 * r / den)%R.
(* Equality over points *)
Definition eqp (p1 p2 : R * R) := (fst p1) = (fst p2) /\ (snd p1)= (snd p2).

(* Characterization of the intersection *)
Lemma interCDr_sol : forall r x y : R,
(interCDr r x y) -> (eqp (x,y) p1) \/ (eqp (x,y) (p2 r)).