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(** * Definition of arccosine based on arctangent        *)
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(** acos is defined by cases so that it is defined in the full range from -1 .. 1 *)
Definition acos x :=
if Rle_dec x (-1) then PI else
if Rle_dec 1 x then 0 else
PI/2 - atan (x/sqrt(1 - x²)).
(** ** Relation between arccosine, arcsine and arctangent *)

Lemma acos_asin : forall x, -1 <= x <= 1 ->
acos x = PI/2 - asin x.