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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
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(*         *     GNU Lesser General Public License Version 2.1          *)
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Local Open Scope R_scope.
(* Proving a few formulas in the style of John Machin to compute Pi *)
Definition atan_sub u v := (u - v)/(1 + u * v).

Lemma atan_sub_correct :
forall u v, 1 + u * v <> 0 -> -PI/2 < atan u - atan v < PI/2 ->
-PI/2 < atan (atan_sub u v) < PI/2 ->
atan u = atan v + atan (atan_sub u v).